I had my wife riding around the back yard last night. She needed to figure out, in a safe environment, how to lift the front wheel and pedal properly over obstacles. As I mentioned before, she's physically fit, just not coordinated when it comes to learning new sports.
First I had her doing the pedal powered wheelie, which was mildly successful. She claims because she weighs less she doesn't have the weight to throw around. I tried to explain that it is just as much about punching the pedals as it is throwing your weight around. She complained for a while and then after a few minutes she started getting the wheel up. She never got to the point of pedaling multiple strokes with the wheel up, but at least she got the wheel high enough to move on to the next step.
I put a 4" log in the yard and had her attempt to negotiate it. Most, if not all mountain bikers wouldn't even flinch at this obstacle but it was tall enough to use for practice. She did well getting the front tire over, but she kept snagging the pedals or losing momentum when the rear went over. I tried to explain in slow-mo what was happening but she was getting confused with my instruction. I told her she needed to pedal to get the rear tire over, but she couldn't have the pedals lower than 4-o-clock when the crank passed over. It's all about timing, and sometimes you have to back-pedal to prevent the pedals from snagging. I take this stuff for granted, it just comes naturally to me, but not so with her. She got over the log 3 out of 4 times, but I think it was more luck than skill. When she didn't make it over, it was always because the pedal would jam against the log or because she had slowed down so much that the rear tire stopped her dead in her tracks.
After some frustration, I had her practice a separate technique. Pedaling between 2:00-4:00 over and over again. She thought this was stupid but again, after a few minutes of trying it she was able to roll around the yard at a slow speed without every letting the pedals drop below 4:00. Unfortunately, she wasn't able to apply this technique to the log. She started snagging the pedals every time once we went back to the log. At that point I think she was done so we called it a night.
I've been finding lots of instructional videos on youtube for mountain biking...but they are all too advanced for what she needs. I think she needs an uber beginner's guide that breaks the skills down into small steps that are easy to understand.