It took me close to 3 years of trying off and on before I succeeded. I was single, had noone else to play with, and had seen a couple videos of women fisting themselves anally, so I tried to do it myself. I was pretty lean and limber then, but I still got pretty sore doing it. Along the way, I twisted and bruised several fingers, my wrist (several times), elbow, and shoulder (several times). I also strained my back a couple times, but I finally succeeded.
My right arm was too muscular and couldn't get anywhere close to the right angle. My left arm was able to bend behind me easier so that's the one I used. I found on sitting on my heels or on my knees and elbows was easiest, sometimes backing up against a desk or door frame for extra leverage. As others noted, I was able to get 5 fingers up to my knuckles, then couldn't go any farther because my arms didn't seem long enough. Almost a year of stretching, bending, and yoga-like exercises added the extra range of motion to finally let me get my whole hand in.
I never got past my wrist by myself, but I enjoyed lots of toys and many many tries along the way. After several years, I finally found my lovely partner who will "give me a hand" when I need it. Whether you're able to fist yourself or not, enjoy trying.