Answer:
Remember that the objective of warming up is avoiding problems when you are working out.
So the first thing that you need to have in mind is that you need to start slow, to prevent any possible harm.
You need to warm all your body, not only some parts of it.
when stretching, do not do it as you normally do, because it will relax your muscles, you should do dynamic stretching, this means that you keep your body in a position where there is no tension, and moving your body to generate tension, then relaxing again, then generating tension, and so on.