Line up the film to where it needs to be on the wall. Line up the screen to the film. Add tape to the outer edge on the screen frame so the tape is half on the wall and half on your screen frame (draws some lines for reg on the tape if you wish). Cut tape so half remains on wall and half remains on frame. Remove the film from the wall. Place the screen back on the wall so the tap registration is lined up. Print. Remember that a large screen will stretch quite a bit while printing.
There's no good way to do this as one person you need 2 people minimum. Usually it's a 3 person job, two hold one printing.
Masking tape or painter tape. Yes it's by eye. We usually bring a wood frame and shims to hold the screen in place so we can mess around but nothing beats the eyeballs for the registration. I have done this where it's screwed a block of wood to the wall to hold the screen but usually that's more work than is needed unless the screen is high up the wall.
Keep in mind that walls are rarely flat so big images can be problematic. Too much orange peel paint texture can be a pain as well.
Just get vinyl lettering cut at your local sign shop. It's super easy to install, looks like its painted on and can be removed without damaging the wall or having to repaint it if you use the proper stuff. It's easier and way more cost efficient than burning a bunch of screens and screwing around with the registration.
My neighbors and I looked into making a vacuum assist suction bar for this. Very strong vacuum, suckers mounted to an aluminum bar with a notch at one edge. Clamp the bar to the bottom of the screen, position screen, vac bar to wall, unclamp screen, ink up screen, reposition on bar, 2 people hold screen, print.
....... the big problem with this was- a vac powerful enough to deliver the right holding power was gonna be a 220 high amp beast. So, we abandoned the idea.
One could make some sort of scaffolding easel on wheels with a movable cross bar.
But all these ideas require a lot of work to achieve, and I think you're better off using Luther's tape method in the end. The physics and mechanicals avenues to go down are just not worth the R&D unless you do this all the damn time.