while I'm no expert on this, what I do know is the FC was not designed for anal sex and unless things have changed the manufacturers do not recommend its use for that.  I would stay away.

And either place it on the penis and then insert into the rectum, or use a finger and insert into the rectum. Always lube the outside and inside so it feels good for both. I ran educational workshops on STI/ HIV awareness in college... Check out the instructions listed in the link below, which also says you can use it with the ring, but ugh I can't imagine that!!!  

I haven't had the pleasure of using the FC there, no greek seekers in a while for me... Should be a breeze though, I use FC's the regular way all the time. Anyone care to try it out with me???