From personal experience, try the following:
Get a face flannel and run very hot water over it. Wring it out and wrap it around your cock for five minutes. This warms you up and gets the blood flowing - important to avoid strains. It feels good too.
Use some lube on the shaft of your cock and insert into the developer tube. Begin with low pressure and build up gradually over a period of five minutes. Pump it up to the maximum that is COMFORTABLE. If it feels uncomfortable it is too much. The maximum time I advise you to use the pump is a 30 minute session. You can use it more than once a day but leave two hours between each session. Any more than 30 minutes and all it does is make the skin puff up rather than actually draw more blood into your erectile tissue.
If you see little red spots on your penis afterwards it means you are pumping too hard and creating blood blisters - reduce the pressure and time used. If in doubt always reduce the pressure - very high pressures can cause damage!
In terms of size gains, you'll see a noticeable increase in your limp size of about an inch after two or three weeks. A permanent increase in your erect size can be achieved but takes far far longer (400 to 500 hours worth!) and is a lot of work. You can expect a quarter to half inch in length and up to an inch in girth.
If you want to maximize your increase, follow the pumping with an hour of stretching exercise, either with jelq exercises or something like this:
http://www.orgasmarmy.com/male-toys/andro-penis-enlargement-system/reviews.aspx
DO NOT USE THIS FOR MORE THAN AN HOUR - it can cut off the blood to the end of your penis if you do! For the same reason, do not fall asleep using it.
A proper session of pumping and stretching will take you a good two hours to complete and you should aim for 4 or 5 nights a week if you are serious. Like I said, it's a lot of hard work. The results are there if you have the motivation though!