Well, this is tough. First it's hard because everyone is different in what they like and second, it's not exactly easy to explain.
With that I will say that if you aren't already you should be open and communicate easily with your wife. After that, when you are having sex, oral or otherwise, pay attention to what she likes, she will move, moan, her breathing will change, and so on.
Now then, to answer your question: kiss her and touch her everywhere, her lips, neck, breasts, nipples, stomach, hips, legs (inner thigh esp.) and then put some focus into her clit/vagina. But don't stop touching the rest of her body with your hands. Okay, don't forget that, use your whole body, not just your tongue. Try licking her clit slowly at first, she should become much more responsive to you by now. Then go with more pressure and a faster pace. Some people like this and some don't you can try "tongue-doing it" her, that's pretty self-explanitory but just in case - it's when you thrust in and out of her vagina with your tongue. Make sure your nails are trimmed verrrry well and when she is extremely excited put one or two fingers inside of her vagina, about one to two inches in, towards the front (think: closer to her clit, farther from her ass). Ok, congratulations you are touching or close to touching her g-spot. Keep licking her clit, with one hand caress her whole body, with the other stimulate her g-spot.
Ok, you now have two options, get her off like this or stop and medical question her - whatever the two of you decide.
I personally like this to be foreplay, but I like to be teased. It makes for a much more intense orgasm.
I think I explained it pretty well but if you have any other questions ask here or pm me, whatever you're comfortable with.