After disabling Nabooris's feet with bomb arrows, you can enter the Divine Beast.
Get the Map
Go forward and kill the Guardian Scout, then go farther ahead and go to the end of the walkway. Turn around and go up the ramp there, and kill the orange eyeball. Go forward into the main room of the Divine Beast. Go all the way to the far wall. There is a ramp up against this wall. Go up this ramp to find the map terminal. In the map, you can rotate the three segments of the Divine Beast.
Activate the First Terminal
In the map, put the cursor on the ring that is farthest away from the map terminal that you just checked on, and rotate the ring three times.
Go to that ring. There is a short wall on one side of the door, and next to that there is a tall wall with an angled ramp. Stand on the angled ramp, then rotate this ring again. From here, you can kill the orange eyeball hanging down under the mouth on part of this walkway, then go around the walkway to reach the terminal.
Get a Treasure Chest
Kill the orange eyeball that is under this walkway if you have not already. Then rotate the middle ring once. After it stops, float down to the treasure chest on a ledge on the middle ring.
Get a Second Treasure Chest
Stand on the edge of the ledge where you got the treasure chest and rotate the middle ring. Stay upright on the ledge as it goes around. Afterward, drop down beside the treasure chest that you haven't opened yet, and rotate the middle ring again. Open the treasure chest.
Get a Third Treasure Chest
Go through the door at the tail end of Nabooris (it has a short tail and a long neck). There is a treasure chest hanging from a chain. Use Magnesis to pull it toward you. When it swings back toward you, use Stasis on it, then shoot the chain with an arrow. Use Magnesis to try to catch the chest after Stasis wears off, in case it isn't swinging far enough.
Get a Fourth Treasure Chest
Rotate the rings so that the three wires are all at the tops of the rings. They will turn green and the wall near Nabooris's long neck will start rotating.
Stand in the opening of the rotating wall and let it take you to the top, then, while staying in the interior of the Divine Beast, float down to the middle of the rotating wall, where there is an opening to a hallway. Go past the rotating spikes. You can use Stasis on one of the rotating barriers to stop a laser temporarily. Go between laser barriers and run toward the end of the hallway before the lasers can force you onto any malice goo, then shoot the orange eyeball at the end of the hallway and open the chest.
Activate the Second Terminal
Go back down the hallway to the main room of Nabooris. Go to the base of the rotating wall and stand in the opening to go to the top again. From there, go outside and go down to the four-spoke wheel. Rotate it clockwise until one of the orange lights is electrified. Then rotate it counterclockwise until the other orange light is electrified. When both are lit up, Nabooris will raise its head and an elevator will come down. Stand on the elevator. At the top, you can activate the second terminal.
Get a Fifth Treasure Chest
Float back down to the door into Nabooris and stand in the opening of the rotating wall. If the opening that you are in does not have a ledge below it that is rotating along with it, go back out and wait for the other opening to come around. Then when you are standing in the rotating opening that has a ledge below it, quickly jump down onto the ledge and walk to the left side, and walk over the end of it when it is starting to tilt upward. From there, jump onto the walkways near the ceiling. Jump across to the green power source, and from there go to the opening in the far wall near the ceiling.
Ignore the metal sliding things for now and go farther outside. There is a treasure chest under the platform that you are standing on now (which is the Divine Beast's tail). Look for ledges below this platform near Nabooris's main body and float down to one of those. From there, go toward Nabooris's body, then float down to the slightly lower ledge where there is a treasure chest.
Get a Sixth Treasure Chest
Float down to the ledge directly below the treasure chest that you just got. Then go into the Divine Beast and get back into the opening on the rotating wall that has a ledge below it. Go back to where the metal sliding things are (see the last two paragraphs for directions).
Use Magnesis to slide the metal thing in the ceiling so it is touching the green wire. Then slide the next highest metal thing to touch the one that is glowing green. Slide the third highest so it is touching the green terminal that lit up when you moved the second one, and slide the lowest to touch the third one. This will make Nabooris's tail move.
You need to lower the tail before you continue, but don't move the metal sliders that you just put in place. Instead, rotate any of Nabooris's rings to cut off the connection from the power source. After the Divine Beast's tail goes back down, go stand behind the tail, where there is an orange light. Then rotate the ring three times (the one that you rotated to cut the power) so that all three wires are on the ceiling again. As long as the metal sliders are still in the positions that raised the tail earlier, the tail will move again. If you stood near the orange light behind the tail, you will be able to use the tail section as a bridge to reach the door into Nabooris's rear hump.
Go toward the door. There is an eyeball blocking your way, but it is closed. You have to walk close to it to make it open. Then shoot it to get rid of the malice goo barrier, and go inside.
There is another orange eye on the wall, so shoot it, then go up the ladder next to where it was. Go to the nearby four-spoke wheel, and rotate it to make the floor down below rotate. Keep rotating it until the wire touches the orange electrode down below.
Before you move on, float down to the hole in the middle of the green circle down below. The elevator takes you down to a room where you can shoot another eyeball. Get back on the elevator and look for the treasure chest and green ball on the ledges beside the elevator shaft. Use Magnesis on the treasure chest to pull it to you.
Activate the Third Terminal
After you open the treasure chest, use Magnesis on the green ball and drop it onto the floor below, but stay on the elevator. At the top, go through the door past the glowing electrode, and stand on the moving platform when it comes to this side. Kill the Guardian Scout on the other side, then if the elevator took you up or down, ride back to the middle and get off there. Shoot the eyeball under the ledge, then check on the Terminal that it was blocking.
Get a Seventh Treasure Chest
Ride the elevator to the top, then go outside and stand on the moving platform to reach the treasure chest at the other end.
Activate the Fourth Terminal
Go back to the room with the elevator and use Magnesis to get the green orb that is near the wall. Then ride the elevator to the lowest level and put down the orb. Shoot the eyeball that is in the corner of the room. Use Magnesis to put the green orb on the orange thing above the bars. The bars will stay open after that.
Go through and kill the two Guardian Scouts. Then go get the green orb that you used to open the bars, and put it on one of the round terminals in the room where the Guardian Scouts were. Then go through to the other room with an elevator, and get the green orb from the floor (assuming you dropped it there earlier). If that green orb is not there, ride the elevator up to find it on the ledge. Use Magnesis to get it. (If there is no green orb on a ledge, you are in the wrong elevator room and need to go to the other one nearby.)
After you put the two green orbs on the terminals near the large gate, go through and activate the terminal.
Activate the Fifth Terminal
From here, go outside to the Divine Beast's tail. There is a ledge on the Beast's right side with a Guardian Scout on it. Go to the left and float to that ledge on the Beast's side. Kill the Guardian Scout on the ledge, then turn around and go into the door above Nabooris's right hind leg. Go through the door into the small room where you can see the terminal on the wall. Rotate this ring once to be able to activate the terminal, then rotate the ring three times to leave the room.
Get an Eighth Treasure Chest
Walk onto the stationary walkway near the rotating wall. From here, bomb the cracked wall that does NOT have a ledge below it. Float there and open the chest.
Get a Ninth Treasure Chest
Get back on the stationary walkway near the rotating wall, and float to the other cracked wall. Bomb it open, then kill the Guardian Scout beyond it, then walk along the ledge on the outside of the Divine Beast (there is a short ramp between some Malice Goo that you have to go up) to reach a room above Nabooris's left rear hind leg. If it seems like there is a barrier in the door, go farther to find an open door. Go into the room and open the treasure chest.
Activate the Main Control Unit
Go through the hole in the middle of the Beast's side where you can see the upside-down Main Control Unit. Stay outside and rotate the middle ring twice. Then you can walk to the Main Control Unit and activate it when you are ready to battle the boss. You might want to go get a non-metallic shield before you activate the Main Control Unit here.