Once you're in the Forsaken Fortress, you will unfortunately be without your sword. Walk toward the stairs and your controller will vibrate and you'll hear a noise. Press A and Tetra will talk to you through the pendant she slipped into your pocket. From now on, if you hear that noise again, just press A and Tetra will talk to you.
Sneak into the Fortress
You now have to get past the guards without being seen since you have no sword to fight them off with. Luckily, there are barrels all around the fortress that are big enough for Link to hide and walk around in. Just press A in front of a barrel to pick it up and hide under it.
Hide under a barrel and go up to the area where the searchlights are shining. There are rupees in this area if you want them. Just be sure to stop moving when one of the searchlights passes. When you're done here, go over to the big wooden doors, drop your barrel, and go in.
The rats steal your money if they hit you. You can get some of it back, but usually you lose around 10 rupees, which is a lot considering that you can only carry up to 200 rupees right now. Stay away from the rats. You can spread bait near the rat hole to buy more bait.
Get the Map
The quickest way to get the map is to get caught by the guards. To do this, head down the hallway to your left and let a moblin see you. I don't think that they can hurt you when they throw their lanterns at you at this point.
You are now in a jail cell. Get up on the table, then jump onto the bookshelf, pick up the vase and throw it, or just set it down with R. Use R to crawl into the hole there. If you press up on the C-stick, it gives you a first person perspective, which makes it easier to navigate crawlspaces. Make your way out of the crawlspace, then follow the path to a chest with the map in it.
Disable a Search Light
Now it's time to disable one of those searchlights. You only need to get rid of the one that is aimed at a place that you need to walk past.
If you would like to get your first piece of heart, drop to the lower level instead of swinging across, then get rid of the barrels to find the switch that opens the jail cell. After getting the piece of heart, let yourself get caught in the next room to get back to the jail cell. Crawl out as before and go to the chest where you got the map.
Use the rope to get to the other side, and take the door to the left. Go through the arch to your left, and go up the ladder there. You'll need a weapon to fight the bokoblin. You can either do what Tetra said and use your shield to make the monster drop its weapon, or you can go around hugging the wall to the right and reach a vase with sticks in it. Break the vase, or use your shield against the monster, and pick up the stick. Swing it with the sword button to beat the monster.
Sometimes monsters will drop a little glowing sphere. It's not a bomb (that's what I first thought.) If you hit it, items will come flying out, including whatever spoils that enemy usually leaves behind (if any.) Bokoblins sometimes leave behind Joy Pendants.
Retrieve your Sword
Once that's done, go back down the ladder, through the arch, and through the door on the right. Now use the door that's sort of straight ahead of you. Go to the end of the hallway and use the door here. Open the chest that is straight ahead behind the barrels.
Use the rope, go through the door, and use the door at the end of the hallway. Once again, use the rope here, go through the door, and use the door straight ahead.
Now pick up the barrel near you, and start walking toward the moblin when it's not facing you. Stay as far away from the moblin as possible as you sneak past. Even if the moblin spots you, you can stop walking once your controller vibrates and you hear the brief drumroll, and, if you're far enough away, the moblin won't see you. When the moblin is walking away from you, go up those stairs, drop the barrel, and go through the door.
Go up the stairs, through the door, up some more stairs, and grab the barrel. Sneak past this moblin and go up the stairs. It takes a while to wait for the moblin to go far enough away, so be patient. When you reach the narrow ledges, sidle across by walking into the wall until the green A in the top right says "sidle," then hold A and use the D-stick to sidle across. After you have made it past the two ledges, go up the stairs until you see your sword. When you can, grab it, and beat up the bokoblin. Then go through the door and watch the scene.