Love a bit of bum fun.
Nothing to add to what has already been said really, other than an experienced girl, and let her lead. Nice and slowly at first, which also increases the excitement / adds to the fun.
Like everything else in this game, dont over think it, just go with the flow, and follow her lead.
You can go from anal to pussy, but MUST change rubbers, which could kill the moment.
I recently fucked a girl (vaginally) whilst she had some vibrating anal beads in, that was nice too. The two 'passages' run close together, so easy to how the girl gets stimulated from to the other. If you are up her arse, a vibro / dildo in the pussy could make a lot of difference to you both, but that maybe for a future experience.
Enjoy - and report back.