After the events at Bowser's Keep, Toad meets you at your house.
Talk to Toad at Mario's House
After Toad talks to you, press the jump button. Your health is already full, but you can check on the lamp on the dresser to rest if you wish. Then press the jump button to get out of bed.
Go outside and talk to Toad. There will be a cutscene.
Go Back to Bowser's Castle
Before you continue, go west and jump onto the floating block to save your game (in the Nintendo Switch version, the game auto-saves when you go between areas, but you can use Save Blocks to manually save as well). Then go southeast from Toad to go to the world map (or, in the Nintendo Switch version, you can press start to go into settings, choose System, and choose "To World Map"). Go northwest to Bowser's Keep, and there will be a cutscene.
Tell Toad What Happened
Go back to Mario's Pad and go into Mario's house. Talk to Toad. After some dialogue, go outside, and go southeast. Toad will bump into you and give you a Mushroom. If you don't know how to use items, you can ask Toad for an explanation. After more dialogue, you can ask Toad for an explanation of Timed Hits (Action Commands in the Nintendo Switch version). If you need an explanation, Toad will walk you through a battle with some Goombas. You won't get any experience or items from this battle.
Afterward, Toad gives you three more Mushrooms. If you want to keep those mushrooms for later, you can go into your house and check on the mushroom lamp to take a nap and heal up.
Outside, jump on the save block that is to the west of your house to save the game, then go southeast to Mushroom Way.