The first step is have something you're interested in doing. If you see there's not a group for it already find some friends or people who are interested in making a group with you.

Next you'll want to approach a LUG Officer and express your interest in making a group.They'll create a slack channel for you as well as announce the new interest group to the large body of members. you may have an alumni assigned to your group as an advisor/mentor

Next you'll want to update the wiki page that's been created about your group