first off you need to obtain level 15 via any method
then on your character screen or log in, one of the two, theres a " The Foundary" button
play around inside this, learn what everything is. you're going to want to hit the "play level from the beginning and run around to make sure the spacing sizes and stuff are all what you're looking too
then when you've created a map, save and publish it, also write down the code so you can find it easily,
then subscribe to yourself so you can bring other people into your created maps