The best way to learn how to strop is to start stropping. All of the advice will help guide you, but just like the first time riding a bike, all of the best advice (pedal faster, steer straight) didn't prevent falling down, or crashing into a wall... And using training wheels really didn't help much either. Once they came off it was a whole different ball game. Same with razor stropping. You can practice with butter knives and old belts, but at some point you will have to try using an actual strop and folding razor, and that's when you will really learn. You may damage a few strops along the way, or not, and dull a razor. But that's just how it goes.