One of my favourite fetishes, it's unfortunately very rare to find it with any actual process. However! There was one story I stumbled onto a long while back on Futanari Palace with very excellent process. It's a very futa and size-centric piece in general, so if that's not your bag, you may want to skip out. It's the second in the series (of two).
I didn't write it (that honour belongs to Byzmunky), and, as it wasn't presented in a downloadable format, I'm loathe to do so now. I do believe you'll need to be registered at FP to view it:
http://www.futanaripalace.com/showth...ht=second+kiss
Enjoy!