You should start by finding a good trainer, because without a trainer you will hardly be able to do stretching. Or rather, you can, but without a trainer it can harm your physique and your body's health. Well, let's say, if you don't have time to visit a personal stretching coach, you can use, for example, online stretching lessons from the famous trainer Flexderek https://flexderek.com/en/stretchingonline/.