Stick whichever side you want although the square is the one i suggest (so the circle is up just to be clear). I dont know if you want to stick it with super glue or something thats why i went with square so i recommend some double sided tape or hot glue.

This is my first model and im also kinda young so i have no idea what the license is however you are allowed to sell this model online but i would like if you gave me a mention on the selling website.

P.S. I recommend you flip the model when printing so square is up since it will be less supports.

Edit: I fixed it after messing with the support, getting cut by my cutters and also making a mess just stick the 2 parts with hot glue or something and you're good