There are some people local here in Dallas that are wonderful fisting trainers/bottoms. I suggest getting involved in the community and learning how to do it safely from someone who knows how to do it.
Like they said, lube/lube/lube!
Start out slowly stretching the sphincter and work your way up to larger toys. If it starts hurting your bottom, slow down, and stretch some more. They can also help you out by training their bottom with butt plugs. You can buy dilation kits that will help with this.
But you need to be very careful. Wear gloves, clip your nails so you don't puncture the gloves or tear their rectum. Vaginal fisting is much easier than anal, but only half of everyone has a vagina...
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There are also a number of sites out there that will educate you on pro's and con's of the various lubes and things you can do. Check out Tristan Taormino - she's been around asses for a long time.