Roller on then the rear straps come down from top either side of quarters so the sheepskin sits comfortably and just on hocks when standing. front strap pulleys clip on to the bit with rope long enough to show a straight line all the way along the side of the horse standing in a stretched neck/long rein position ie head relaxed and dropped but not "on the bit" then continuing down in the straight line with no tension to clip between legs.
Never do too tight as your horse may rear. If they are not keen on the back part to start (a lot dont) then fit correctly then move it back up to sit on their bum and as they work it will gradually fall down and most find this a more acceptable introduction!!
Also dont expect or ask any horse to canter with a pessoa for the first 6 weeks of regular use!! Lots of people do but this is the time frame recommended.
You can once fully established working long and low move the clips from between the legs and instead clip to the horses side (engages quarters) or even higher if you know what you are doing! However most people only use position 1.