Also there is an excellent article about wing clipping here
that explains how and how much.
May I suggest that you try first without toweling him? My U2 (Taz, male, 10yrs) has always been toweled by his former owners, but I have found that working with him I can clip his nails. He will watch carefully, but allow it.
One nail at at time, and just a little at a time. But it is soooo much easier.
And wing clipping is coming up. I want to do it this time as I disagreed on how it was done last time. I strongly feel that the wings must be clipped symmetrically for the birds comfort.
So I will try to let him present the wing and then cut one or two feathers at a time and see how he reacts, but chances are that he will let me.