For this, start the game, enter a username, then click play on whatever featured world is on your screen after making your name. Now either play it until you're done or just leave the world.

After leaving whatever world your were in, click the explore tab on your screen, click the search bar in the top right and type in whatever you want and hit your enter key.

After (or before) successfully searching something, like any world on your screen via the explore tab. Then exit back to the main screen with the featured worlds.

Click the create button on your screen from the main screen with the featured worlds. Press your ESC key and click save and quit. You will be brought to a screen with your worlds, simply click upload and you're done with this game.