I only have experience in topping... and good advice has already been given here.
Before I go in I prefer to get her relaxed, lubed and ready to receive my cock. I start by gently stroking my tongue around her anus and poking inside with the tip of my tongue while swirling it around. I generally get really turned on and begin to salivate quite a bit, producing more than enough "lube."
When she's ready I like to slap the head of my penis on her anus and gently press in, sometimes I'll spit on it some more to get a little extra lube and let it slide in. I just work it in little by little until its sliding freely and begin going at it balls deep.
