Question about when it is best to stretch.

I have been going to the gym for about 5 months. I have not done much cardio. i do not not have time to do cardio and weight training in the same session. But it needs to happen. I have been thinking of doing cardio after work and weight training before work. When to do my cardio is not the question. It's when to stretch. I already do active stretching before weight training. From my understanding active stretching does not help much with flexibility but helps prevent injury. I do not have much flexibility in my legs and I want to fix this. From what I have read, static stretching after muscles are warmed up like after cardio is the safest thing to do and stretching with cold muscles could cause injury and could reduce gains. First is this true or have I read wrong. Second how hard should I be stretching to improve flexibility. I am willing to do about 20 min of stretching.