beforewisdom, the best way to build up to a specific bodyweight exercise usually involves an elaborate system of progression from less demanding exercises to the targeted one.
In case of pistol squats, this may involve progressively increasing ROM: you start with one leg box squats with a pretty high stool/box etc., and then gradually lower the stop, and get your non-working leg in the forward position. If flexibility is an issue, you might have to add specific stretching exercises on top of that.
In general, these progression schemes involve either increasing ROM, or leverage (e. g. in case of pushups: wall pushups > decline pushups with hands placed on chairs > knee pushups on the floor > regular pushups - works with one-arm pushups as well).
A different question: Does it make sense to pursue the long-term aim of becoming able to execute a difficult BW exercise, such as pistol squat, one-arm pushup or pullup?
My advice would be to respect your personal build - individual factors such as height, bone structure, weight distribution, leverage etc. play a huge role when performing these feats. While some people are able to master difficult BW moves almost instantly without much specialized training (especially short people with a "wiry" build), taller people with unfavorable leverage have the odds stacked against them from the start, and may easily get hurt trying to accomplish something they are just not built for.