Make the object

Go to the battle editor, and use the terrain editor to make tall terrain, but make sure there is enough terrain to accommodate the size of your object, and when you're done making the terrain, click the checkmark on the top left and a notification should pop up on the top of the screen saying "Do you want to update modular buildings?" , when that notification pops up click "yes" and put your object on top of the terrain

Now, make the terrain go all the way down, and after that click the checkmark on the top left of the screen, and a notification should pop up on the top of the screen again saying "Do you want to update modular buildings?" , when that notification pops up click "no"

Now you know how to make a modular building float!