The following photos will, I hope explain how I convert my Jackets.
This picture shows me measuring the the jacket to length, for me it is twenty four & a half inches plus one & a half inches for the seam. It will vary from person to person, I measure from the bottom of the collar of the jacket to the bottom to the sewn part of the pleats on the kilt. (26" for me)

cut it to your size(26" for me). I have already cut this jacket. Dont worry about the sporron curve at this stage, just cut the whole length of the jacket, measuring from the bottom of the jacket up to your correct length along the jacket to give the excess.

This photo shows me marking out the sporron curve, I do this free hand until it looks right. I start from 1" below the button & draw to the finish length of the jacket( 24 1/2" for me

Measure one & a half inches below the curve to give a seam. this matches up with cut length of the jacket ( 26") for me. Cut the excess material from the bottom of the curve

It should look like this

Remove the pocket flaps & move them up so that they just cover the opening
Reattach them in this position after sewing up the opening ( they become false pockets)

Some of the material off the bottom can be used to make epualets if required
The finishing off, turn the seams over to the finished length along the bottom of the jacket & pin, leaving the lining hanging down. Sew along the jacket then cut the lining to suit.
The curve is a little more trickey, it is better to use bias binding, attach it to the outside of the jacket along the curve, then fold over & cut off excess material leaving only 1/4 " & then attach the other end of the bias binding to the inside of the jacket, this will give you a much better curve.
I know its long winded but I hope it makes sence