Wow, this is a busy thread!
Yes, it is a sight thing. They can only see down and to the sides. It seems kind of cruel, but I did not want to get rid of my birds, and nothing else worked for me.
It would be difficult to put on without 2 people (at least for my birds that are not used to being handled). The site I got them from recommended some kind of pliers, that cost much more than the peepers (maybe C-clip pliers?). DH has every tool, so we didn't buy them. Maybe needle-nose pliers would work,- you are using them to widen a springy piece of plastic, with tiny tabs that fit in the brid's nostrils. They should be able to breathe through their beak when correctly installed.
We put them on the leghorn in the middle of the day. Not only did she freak out, but every bird freaked out grabbing at her peepers like they were food. They pulled them off of her. We went in the next night and snatched her off the roost, putting them on by headlamp, put her back on the roost and she went to sleep. In the morning, no one even noticed, and the leghorn was scratching around like everyone else, but not pulling feathers!
I know there are people on the forum that outfit every bird in their flock with peepers for the winter, and take them all off in the spring when they are not cooped up.
Good luck with them! It is a frustrating problem.