Lots of lubricant...work them up slowly one finger at a time...when your four fingers in keep at it gently...lots of clitoral stimulation helps (presuming this is vaginal fisting)...pull your fingers partway out and hook your thumb behind them..again continue to finger this way untill your upto your knuckles...then its lots of slow twisting to get the knuckle in...then enjoy ......hope this helps :)
I agree with the previous poster, but just wanted to add: Make sure your partner really wants you to fist him/her. If s/he isn't really into it s/he may tense up and then you either won't be able to get in, or it will be too painful for him/her to endure let alone enjoy.
Yeah just fist her good and make her scream we both know she will love it! Hell yea I know I do!
Just shove your hand in there and don't worry if it hurt her