OP, there are generally two types of sedative used on horses - one is an owner administered paste which you squeeze into the horses mouth.
Do NOT let any of your barn mates or trainer talk you into using one of these for clipping - as described by another poster when the fear of the clipper hits the horse that is feeling 'a bit dopey' fear can often increase because he feels more vulnerable. The adrenaline will override the sedative.
The other is the vet administered injection. This is an expensive option and must be administered by the vet who will then remain on site to check how the horse responds.
If you haven't clipped before (apologies, this isn't clear from your post?) then even a vet-doped horse will be hard for you to do a good job on because he still won't be standing statue-like and as you get towards the head, or wherever the sensitive areas are, he will be objecting despite the drugs.
Believe me, my old mare was a cow bag to clip and the first time I did it, under sedation, she looked absolutely awful...........
BUT she did get better through firm (not rough just steady) handling and lots of desensitisation. For her I couldn't just do the desensitising I also had to stop her throwing her weight about and quit.
Taking all the hair except the legs off is a Hunter Clip by the way. Leaving the face on is possible (sometimes necessary) but also looks a bit silly (again, I speak from experience