www.hundredoaksinc.com is a wonderful web sight. Marti Friddle is a very accomplished sidesaddle enthusiast and this is where I found the following information:
Fitting the Horse -
We purposely picked a horse with very low withers for this shot. If your horse has higher withers you won't have a problem finding the saddle pocket - the hollow spot just behind the withers. On horses like this one however, you have to manually feel for the pocket.
Once you've located the saddle pocket, use any type of wire - coat hangers, baling wire, very stiff electrical wire - whatever you can shape to your horses' back and shoulder.
Be sure to extend it about 20" down the shoulder, because Hunt Style sidesaddles have a point that extends that far. If you send us a tracing that doesn't show the shoulder, and you want an English sidesaddle, we won't be able to tell if the point will dig into your horse - a situation you DO NOT WANT!!
Try to get the shape of the wire as close as possible to the shape of the horse, then trace it onto paper. You should have a shape something like what's shown in the upper right hand corner of the photo. WE NEED THE ORIGINAL TRACING - not a reduced copy! When we receive it, we'll hold it up to the saddles in the shop to try to find the best fit.
Mail to: Hundred Oaks, Inc, P O Box 886, Graham, NC 27253