If you want to make truly realistic eyes, I'd recommend buying the video
https://www.stanwinstonschool.com/videos/making-a-monster-eyeI haven't tried that method myself, since it's outside of my price range at the moment.
Another method which I've seen is to print off the iris, glue it on ping pong balls, add red yarn, and coat it all with clear nail polish.
Another thing that could be done is to cast eyes using clear resin to be hollow and then paint them from the inside. Or have white eyes that you paint and then put in a mold full of clear resin.
As for me, my current method is using ping pong balls, a compass, colored pencils, and coating with mod podge.