A warm-up before football should take at least 5 minutes. You should have a light sweat, your heart rate should be up, and you should feel loose and ready to go. The most common mistake athletes make when warming up is not warming up enough. Athletes don't want to "tire" themselves out. Many believe that when you start sweating and breathing heavily, you're wasting energy. All your body is doing is adapting to what you're about to do. Therefore, a good warm up allows your body to wake up and get ready to go. Not warming up properly can result in injury.
Static stretching is very old school. This is when you hold a stretch for a period of time. Static stretching is what many athletes like to do before football workouts, and there are enough studies showing that static stretching before physical activities is not the best for you. It is not good for you to stretch muscles too far. You are just leaving yourself more susceptible to damaging the muscle.
For this reason, many have adapted to the dynamic stretching method. This is when you hold stretches for a short period of time and are constantly moving. You force your body to get warm and will often generate a sweat. It stretches your muscles without over-stretching them. Dynamic stretching prepares your muscles for the movements you're about to do.
It is great to do static stretches after your workout is complete. Static stretching after workouts and even at night before you go to bed is great for you. It allows you to be more flexible, mobile, and helps you recover after workouts.