What I did for my first rifle was take a picture with my camera of the wrist / buttstock profile from a book, get that photo on my computer, and then open it in Microsoft Word. I then set the paper size to tabloid (11x17) and orientation to landscape, and toggled on the rulers to be visible. I then drew a line at the length of my desired LOP using the ruler in Word, then scaled the photo of the buttstock for that line to reach from the buttplate to trigger. I can't print 11x17 paper on my home printer, so I had to splice together the two sheets of 8.5x11 paper that were printed. I'm sure this method will work with most any of the photo editor software available.
Like others have said it's difficult to translate a 3d object to a 2d photo and retain perfect proportions, so I used the pattern I printed more as a guide to trace onto my stock than as a hard and fast pattern. I glued it to some posterboard so it would be a little more rigid for tracing, and had to utilize it a few times during my build to re-establish lines after drilling the ramrod hole and installing the buttplate. A more proficient builder would probably be able to start with one plan drawing and keep it throughout the entirety of the build, but I'm far from proficient and probably a little lucky that I was able to end up with something that resembles a rifle while chasing a moving target.
I'm doing the same initial photo enlargement for my second rifle to once again get me started in the right direction for a Lancaster profile, as well as helping to determine the size of the patchbox and engraving in relation to the stock. This time though I'm going to take a lot more time establishing the lines for the barrel channel, web, and ramrod hole on my plan drawing.