While you're eating her out go down to her anus and start licking it and ask her how it feels. If she says she likes it then gently stick a finger in there. ask her again how she's feeling...... work your way up there buddy. It may take time but just be patient. Don't be pushy about it. Be nice cuzz I know if something went up my ass such as a dick, iI'd be in extreme pain and if someone was being pushy about it then I'd say "forget it."