Not a big problem. First back off the nut so it is even with the end of the thread, that gives you a bigger surface to bang against. I use a brass drift and a big hammer, but a piece of wood and a plain old carpenters hammer will work. Put the drift or block of wood against the backed off nut. A couple of sharp taps with the hammer should break the pin loose.

If you do damage the cotter pin (It was always recommended not to reuse one anyway) they are available, check eBay and Amazon for 9.5 mm cotter pins. The 9.0 mm is for French bikes you do not want that one.

I usually tap the pin back in the same way I tapped it out then tighten it until I think it is a bit too tight. That is about right.

BTW, the chain should have about a half inch of slack in it. I keep seeing chains pulled up tight in photos here. That is not a good idea as it puts a stain on the hub. It may also explain all those reports of a ticking noise when pedaling that I do not hear on my hubs.

Nothing difficult about this at all, I was doing it when I was 9 or 10. I have always liked cottered cranks they do not give problems like cotterless cranks sometimes do .