How To ? Convince your wife to take "That" in her mouth like this.
You asked about convincing your wife but did not tell whether you do the same thing for her or not. If you do oral sex with her which she really enjoys, don't force her, she will automatically be ready to give you oral sex. But if you've never done it, why expect it?
Whenever having oral sex, keep in mind that neither of the two has any sexually transmitted diseases. Also keep in mind that don't stick to just one part. Pay attention to the whole body, later it has to be concentrated in one place. Also be sure not to gnash or bite hard. Biting with the lips may go very lightly, your job is not for pain but for pleasure. Have fun while working on his parts, don't treat it like a chore.
Even when the wife is ready, keep in mind that she should not have any problem, otherwise she will hesitate to take it again.