I would imagine you would need to be fairly flexible to self fist, but as muse said, everyone is different so don't stress.
Having a partner doing the fisting would be a lot easier, even then though, depending on hand size, pelvic girth, etc it may not be fully possible. Plus as muse also said. "Lube Lube Lube, no such thing as too much"
I would also recommend taking it slow and easy. Sometimes it takes a while for the vaginal muscles to relax and /or stretch enough.

Hope that puts you mind at ease

Regards
Paul