Give it about 20 minutes before you plan to start. Do not let him see/hear clippers or anyhting else that might stress him (horses arriving/leaving the yard etc) before he is 'under' if you can help it, as if the adrenalin levels spike before the sedalin has kicked in he may well fight it and then you are 'stuffed'
Once its kicked in he should really be sleepy and chilled - you may want his head near a stable door or something you can prop it on so you can clip under his throat if you haven't got a helper to hold it up - some zonk out so deeply their chins are on the floor.
On the other hand, I clip one for a friend (14hh NF) and we started with a third of a tube and ended up giving the lot and STILL we couldn't get her done as she fought it all the way
She is now sedated by the vet for us to clip her...