Two options:1.) To start a "formal" contest, like the ones featured on this site, contact the advertising department. Their info and email address can be found here: https://www.instructables.com/advertise/2.) If you're interested in starting an "informal" contest, go to the forums (Community) and post details there. See this one as an example: https://www.instructables.com/community/LEGO_CONTEST/Option 1 is going to get your contest the most publicity. I also suspect that it's not free when you pursue option one. That said, with option 2 I personally would be a little leery about entering a contest posted by some random user on the forums. You never know if they will abide by the rules they set or even distribute the prizes as promised in the end.