If you haven't managed to get to the vet yet, use the same treatment you would for a child
Cool boiled water, sterile pad dipped in it, wipe from the corner out once, change the pad or turn it to a clean bit and repeat - to remove pus, crustiness etc.
I had to do this with a quail, and their eyes are tiny. Didn't need the vet - it was just dust from their bedding that had got encrusted