One, buy an expensive clamping tool which will apply even and heavy pressure to the pouch cells to keep the chemistry evenly distributed and working effectively. Something like the Band-It tool.
Another option is to use hose clamps. Hose clamps aren't nearly as strong as the Band-it tool, however, there are a couple of advantages. First, no expensive tools needed. Second, they are removable. The Band-It tool's clamps need to be cut off and cutting a band that is under great pressure isn't very safe. Third, they can be bought fairly cheaply.
I found the MakeAClamp system online. It's a very flexible system in that it allows you to specify how large you want the clamp to be. The system is based on clamps that can be re-positioned and bands that have notches cut in them. I bought one kit which contains 3 clamps and 10 feet of banding material.