Make sure the peepers are centered in the nostril, not too forward, not too far back. The peepers will allow them to see down, up and to the side. They will not be able to see directly in front of them, especially to peck or pick at something. However they will still be able to walk toward you and see to the side and such. It takes a couple of days for them to really get used to working with them on, eating, drinking, etc... If they wear them long enough, they can actually learn how to get around the sight issue and still peck and pick!! LOL But we are talking years. (I had a mean hen wear them for about 4 years. She got so it was as if she wasn't even in them.)
Good luck!