I honestly think the best way to learn to shave someone else is to....well, try and shave someone else! I think that would be the best experience for you, as long as you have some very trusting friends that would be willing to put their face on the line!

You want all your groomsmen to get a straight shave before the wedding; have you considered finding a nice barbershop that gives straight shaves? You could always just treat all your groomsmen to a nice straight shave at a shop.