Even though it might seem like playing for the duration of a football game is enough exercise, it is extremely important to include warm ups before each game; if you don't, you can end up causing yourself unnecessary injuries.
Ideally, you want to start your warm ups around thirty minutes before the game begins, with the first exercise being jogging for around ten minutes. Starting with a gentle jog and increasing to a slightly faster pace towards the end of the ten minutes, is a perfect way to increase your core and muscle temperatures to the level at which they need to be.
Following on from this, you're now ready to do some static stretches, you want to spend about five minutes alternating through various stretches to include all key muscle groups, and because your muscles have been warmed up enough from the jog, you should not experience any discomfort during this stage.
Next, you should spend another five minutes imitating the actions you will be doing during the game, obviously for this you won't be using the ball, but you need to pretend that you are in the middle of playing. So for this, you want to practice jumping, high and side kicking, squatting down and leaping back up, essentially all the football actions you will be doing throughout the game; think of it as a practice for your body so it is ready for the real thing.
For the final ten minutes you now want to include a football into your warm up, your body and muscles should now be fully stretched and warmed and you are now ready to have a quick practice by way of functional activities. You should do as you did in the previous stage but just with a ball, practice headers, passing, side kicks, air kicks, long kicks, side steps etc; in addition to this, you should also include some quick bursts of sprinting, stopping for a second, then going again. As much as you may feel like you will want to conserve your energy for the game, you will be better off in the long run and play to a higher standard if correctly warm up beforehand.
It should also be noted that while you don't need to do any more vigorous stretches or warm ups during the game, it is beneficial to take a few minutes during halftime to do a couple more static stretches on your major muscle groups, this will help you during the second half of play.