When I get someone in class who's problem is the fear of their butt being higher than their head I have them start with something MOST people are somewhat familiar with, a regular playground over-the-head somersault. After they do a few of those I start changing things one thing at a time. First I put the feet in the proper position for a kneeling roll. They do a few of those on both sides. Then I put the hands and arms in the proper position and have them "push against the earth". I tell them to keep that feeling of pushing through the entire roll. I position their head and have them do a regular somersault. The difference is that with their feet, hands, and head postitioned the somersault goes from rolling down the spine to rolling across from shoulder to hip. Occasionally I'll get someone who needs a little extra help and then I'll do all of the above and I'll cradle their head through the roll and help lift their butt over (like a gymnastics spotter). This method doesn't usually produce "perfect" rolls but most people are doing a safe roll on the first day by the time the rest of the class is finished with their warm-up ukemi.
I've had pretty good luck by starting with something they are at least familiar with and to start changing and tweaking it slightly into what we're trying to do with our rolls.
Hope all that made sense,