I've never engaged in fisting so I'm a poor respondent for your first question, though I'm sure there are many on the forum who can give some pointers. Beyond that, there are a whole bunch of discussions on here about fisting already that are undoubtedly full of tips/tricks for fisters (you can find those by doing a search for "fisting" and then chosing the "discussions" tab.
As for anal fisting, it can hurt if it's not properly introduced, or rushed, the body is an amazing thing and the anus can stretch to receive a fist in the same way that a vaginal cavity can, though it probably takes more time. There are threads about that too that come up when you do a search for fisting