Pump it up until you're at the highest vacuum level you can comfortably obtain. To obtain the best results from pumping you need to go for longer periods of time, an hour or more. If you have more time, go for more. Your increases will be larger. As your penis gets larger, increase the pressure some more, again, only to the point where you are still comfortable and not in any pain.
Most men are amazed at the size increases which are possible.
I read one faq which discussed creating your own pump out of PVC pipe. I wouldn't recomment this at all. If you can't see your penis, you're unable to monitor progress, or see if there is any discoloration. You need to be able to watch so that you can be alerted to the need to stop. Plus it can be quite pleasurable to see your penis double or more in girth, and gain an inch or two in length.