The first thing that you want to do, is to go on and study cavemen. You need to figure out what they looked like, what they wore, how they dressed and how they are supposed to look like. This is going to require you to go on and do a lot of research. However, this is research that is going to help you quite a bit. This is why you should not waste any time and go on and look up books and the internet to study cavemen and their appearances and habits.
Once you have gone on to figure out just what the cavemen looked like, you need to go on and draw a normal caveman. This will be your reference. Now go on and use this reference to create alternative versions of cavemen. In all these alternative versions, go on and exaggerate the lines and expressions on the caveman as much as you can. Draw him in a funny situation, where weird but funny things are happening. Keep going till you have quite a few versions, and a number of different expressions and poses.
After you have gone on to do this, go ahead and present your artwork to a bunch of friends or other people. They will be able to take a look at what you have drawn and tell you if it looks like that what you were trying to make. If they say that it looks just like a comical caveman, then you have won. However, if they say something else, you need to start from scratch once again.