Im not entirely sure of what the different straps are called so I named them myself :) I've done a pic of it :)
First thing you need to do is attach the breeching straps to the roller. These are easily recognised by the fact that they are the shortest ones and also the only one that is one single piece of rope attaching to both sides of the bum strap (lol gotta love these made up words :))
Its pretty easy to get it right really :) the only straps left to deal with are also attached permanently to the bum strap. One on each side. It then runs up the side, through the side ring of the surcingle, attaches to the bit and then runs down and attaches to underneath the surcingle. There should be either a ring or a sewn in piece of material there to attach it to
Then obviously, you do the same thing on the other side :)
For the first time I put it on the loosest strap just so that he could walk around and do a little lunging while getting used to the new bits and pieces. It allowed him to move his head if he wanted to but now he is used to it I will set it at first for about the same length as it would be if I were in the saddle...nice and soft but still with enough pressure to let him know you are there :)
Be careful though. This could be where a lot of people go wrong with it. Take it slow and think about how tight you are making it. Theres no point in trying to rush it and have him/her putting themselves in a position where they are going to be sore from it. Applying a little logic is all that is needed. I didnt get a trainer the first time and we did really well. Which kinda surprised me because its only recently that I've started using gadgets of any kind and I usually have little patience for figuring things out lol but anyway, just take it all slowly and only tighten it more or head on to the next phase when they are ready.
Tips - I would suggest trying to leave it on the surcingle if you can. This will depend on which type you get but I got the pulley type and the pulleys don't fit through the rings on the surcingle so you have to unattached the side straps from the bum strap run it through the side ring going from front to back to avoid the pulley that is on the clip which attaches to the bit.
If you get confused by which rope is which, lay it out like I did in the photo and you will soon see how it goes
Hope this has been helpful for the few people that I know were interested :) if anyone has any more questions feel free to ask and I will try and answer them. If I don't know the answer I will go and play around with it until I figure it out