Most of the commercial screen printing exposure units have a vacuum blanket made of rubber. The screen is positioned on the unit and then a vacuum blanket is closed and clamped down. A vacuum pump then removes all the air and the rubber molds around the screen and that put pressure down on the artwork attached to the screen. I have also seen some home made versions that look like they work pretty good also.
I haven’t built a vacuum blanket yet for my exposure unit so I hold it all down with weight. I bought a piece of 1/4″ plate glass from the glass shop and I put that down inside the screen frame and then put a piece of Styrofoam on top of that. Then I put a weight plate and a water jug or a full laundry detergent container on top of all that to add more weight. It works good. I am going to build a vacuum blanket some day and that will replace the weight. So, I made a video to help explain what I tried to describe above. 🙂