I think it is not hard to apply for a signature campaign, just go to the marketplace -> services or to the marketplace of altcoin and there search for a signature campaign which allow Jr. members and there you will find the rules to apply , just follow that , or go through the posts of other members and apply like they have.
Yeah every campaign got rules in the OP, so read them and make sure you understand them. If you don't you can always ask a campaign manager.
They are there to guide you and existing participants to make it an success.