We tried fitting our convertable carseat and we couldn't get it right either. It was SO worth the $30 or whatever to get it fitted. I'm glad we tried first as we understood it then and we both went and watched as it was professionally done so we understand how to do it better now. Since then when take the seat interstate so in and out of our car, we've always done it ourselves and we do a pretty good job.

ETA: I turned my son forward facing at 7 months. He was crawling and sitting by himself and met the weight recs. But I only did it as I couldn't get the seat to reach the anchor in our hire car with it in the rear facing position. I could have bought an extender the next day I guess but I never did and when we got home we left him as I don't think he would have been happy about rear facing again. That said, I wouldn't have done it by pure choice that early and we'll wait longer next time.