Lots of lube, lots of patience-don't force more than you can stand. Some females before childbirth cannot do it! It may help to use a latex glove (ie, less friction).
Also, there are some techniques and positions. you can also involve a females with a small hands. That's the safest route
Note: while I have not self-fisted (my torso is too long, my arms too short), I have fisted females vaginally.
Let me look this up for some resources; I'll be back!
A lot of lube
Boy. My Old girl friend did It and she wasn't as tight after