Hi,
I've been fisted by women. Their hands are smaller, but I've been trying to find a guy to try to get in me too.
Getting fisted properly is awesome, finding someone you can relax enough with is the hard part. Being totally relaxed, eager, and ready is critical.
Being ready...clean, most important thing. If you don't have, get an enema bag (hot water bottle), or plumb a shower shot type unit to your shower.
I smoke pot, helps relax greatly, I also use poppers throughout the event.
I get in the shower, wash my ass real good, then fill the bag all the way. Use very warm, but not hot out cold water so you don't burn or cramp. A 1/4 teaspoon of table salt in each bag is reported to help keep you from drying out inside.
I apply a small bit of Vaseline to my hole with my finger...this is where the good feelings start...
I then slide the nozzle of the bag in my ass.
Release the clamp and let the warm water fill me up...as the bag nears empty, I pull the nozzle and wash it off. I then take a couple hits of the poppers, it relaxes you inside and lets the water get deep. I then refill the bag, and really fill myself up...
I hold this a long as I can, taking that time to fully shower. When its been 10 minutes or so, or I can't take it, I hit the toilet and let go. The emptying can take 20 minutes.
Once I feel empty, I repeat the two bag clean out, but this time I push the nozzle past that second sphincter about 6 to 8 inches in. After this it's usually a few more single bag rinse outs, until the water that comes or is totally clean.
Relax for a while after all this, get hard, stroke yourself, get nice and horny...work your abs, and rock side to side for a bit to help move things through you.
Honestly, take a look at a diagram of your physiology, know where the hand is going, make sure your Top has this knowledge. The rectum is where the hand will make it's first fully enveloping stretch...this is the hardest part to handle the first time...if you've been prepped with small toys to large, taking your time, and maybe multiple sessions, you'll finally be ready for the fist.
Tools to have on hand...gloves, use nitrile or latex if you're not allergic. Lube, for fisting and big toy play, I like Crisco, for other stuff, silicone. Crisco comes in small cans (get two) good for one use, and cheap! Towels, a few towels to spread over the bed for a play area, a couple for the Top to wipe up with. Any toys you want to experience in your ready ass...again, silicone material makes a great, big toy...veggies...bottles...plugs...beads...
The Top must trim and file their nails super smooth, and no rings.