We have no rules for how you make the model. The software is completely up to you as well as your modeling methods (feel free to use any tutorials you find). This subreddit is to help encourage people to improve and practice their modeling, so following a tutorial is completely fine as long as you are learning.

One other thing, you do not need to rush and post something the same day, post it whenever you have something ready. There are no time limits on challenges.

If you have any suggestions at all, including ideas for future posts or advice on how to make the sub better, please message the moderators.

Sites for challenges/inspiration/help:

Polycount
CG Society

Other places to post:

/r/passtheblend

/r/CrossView

/r/computergraphics

/r/lowpolytutorial

Another note: as cool as Sketchfab/Ibb.co/Pintrest are, links to those site get caught in the spam filter, which automatically deletes comments containing them. If you want feedback, please use a Reddit preferred image host such as Imgur.