Practice rope ascending on a top rope
The image below is from the outstanding online big wall climbing course from VDiff climbing. It's a great course and quite a bargain; highly recommended!
Want to climb a big wall? Dialing in your rope ascending and cleaning techniques is key.
Here's a way to reduce risk when practicing these skills.
Set up a solid top rope anchor. On that same anchor point or something very close by, hang a single fixed rope.
Now, as you ascend the fixed rope (green) you're always backed up with a proper belay from your partner on the top rope (red). Now you can practice things like cleaning a traverse, leapfrogging past fixed gear, cam jugging, lowering out, maybe doing a mock lead, and other big wall shenanigans.
If you screw up anything on the green rope, no problem! You're backed up on red.
(This also applies to non-bigwall flavors of rope ascending, like prusiking for crevasse rescue practice.)
image: https://vdiffclimbing.thinkific.com/courses/take/big-wall/lessons used with permission