Hi Jim. The bags I bought have a "one way air vent", that allows air to escape out the end when you roll them up. You don't use a vacuum. You put the clothes in, and seal it (there are directions on how to seal it, it has to be done a certain way), then you roll it up, (like you roll up a beach towel), and as you roll it up, it vents the air out the one way air vent. Be sure to leave enough room in them to properly seal them. Try not to overstuff them. I have used each bag several trips, in am still using some of them. Good luck!
Enjoy Alaska, I cruised her last year. Lil' Lori