Affinity Photo is a non-trivial tool that requires considerable study and practice to master, indeed, to do even some simple things.
When novices like me inquire about how to do something, it is more helpful to give specific steps. Instead of instructing:
"...copy + flip a (the?) healthy eye..." which is something I actually need to do but couldn't using that particular hint. I had to do more searching to come up with even part of the process: use the freehand selection tool using a feather of X and anti-alias, select the good eye, copy it using shift-command-c. Now, flipping it is beyond me as I can't find directions for flipping a selection. My experiments flipped the entire layer. And now, there are 'eye-shaped holes' in the layer I just worked with. My 120 year old photo of my uncle remains blind in one eye - and has become holy.
I am hoping specific detailed instructions are exempt from the egos of the masters on this site.