First you want to lube up your pointer finger.
I think we both know where this goes, and he does too.
Stick it in. Move it in and out, make sure to use lots of lube so that it lubes up the inside of his anus as well.
Move it in and out for about a minute. And really think about whats happening. This in and out motion is what a mans dick does when he fucks you. You are feeling on your finger what he feels on his dick. Its just WAY more pleasurable and intense for him.