First, I agree with everything Steven said. If you do want to build up to the pistols directly, you might also consider the following posts/articles:
Originally Posted by Steve Maxwell
January 05 2006
In almost every case that I have seen, knee pain while performing pistols is caused by mis-alignment of the knee, foot and hip. This is caused by several factors; inflexibility (achilles, groin, hip flexors, hamstrings) and a simple lack of strength in the involved muscle structures. In almost every case, the problems were solved by teaching the person to hold the bottom position of the pistol for extended periods of time (over a minute) while maintaining proper alignment. Proper alignment includes, heel flat and engaged, middle of the knee cap lined up over the index toe and the head forward over the knee. Hold onto a pole or rail for time while keeping perfect alignment. Keep increasing the timed holds and slowly wean yourself off of using your hands for balance. When you can hold the bottom without using the hands for a minute, you will be able to do 3 to 5 pain free pistols. Remember, it must be perfect alignment. Steve Maxwell
Pistol Power: Mastering the One-Legged Squat
by Steve Cotter (WFS)
The above article includes a troubleshooting section that deals with inflexibility, knee instability, etc.
Pistols are also discussed here by Jim Bathurst.
Still, nothing builds the requisite strength like the two-legged squat. If this is a bit of info overload, I would just stick with Steven's advice.