The first step is to meet other writers who are interested in a writing group. I've met people in writing classes and at local SCBWI events. So be friendly to people because they may just help you along the way.
1. Set a schedule and stick to it. I go to one writing group that meets on Monday nights. I go to another writing group that meets every other Tuesday. This is good because I know what I'm doing every Monday night and also every other Tuesday. The schedule won't always fit for everyone and that is OK.
2. Show up. The only reason I miss my writing group is if I have work. One of my groups is small, 3 of us, so attendance is pretty important.
3. Make the time productive. In my novel group we don't always have things to workshop so we usually set a timer and write for an hour.
4. Take critiquing seriously. OK it takes time to read a novel. It takes time to notice good writing in that draft and it takes time to find areas that need attention. Take the time because someone will do that for your novel one day.
5. Give quality feedback. I like to state the things I liked, the positives, first. For other things I ask questions. Remember that drafts are sensitive things. And behind every draft is a writer with feelings. Be careful with them.
6. Read. Every time you read a book/essay/short story you are reading something that was once someone's rough draft. Reading can help your own writing and it can help your critiquing skills. Seeing what successful books did well can help you in your own work but also might be able to help you notice great writing or areas that need work in your writing partners writing.
Do your belong to a writing group? What are your tips for successful writing groups?