Search for a world you would like to buy. (We will be using Mineville Pool Party in this tutorial.) Look at its price first to ensure you have enough minecoins to make the purchase.
Minecoins store menu
Purchase the world, and then tap Download. Essentially, you are downloading a template. You will use this template to create the actual world you will be playing on.
If you are still on the world’s Marketplace page, you should see the Create this World! button. You can either tap that, or navigate back to the main Worlds screen (from Minecraft’s homescreen, tap Play), then tap Create New. You should see the world you just purchased under Available Templates.
World creation menu
Select that template, adjust the game settings, then tap Create. Have fun!
Mineville Pool Party world