Making a pattern from an existing garment

[Sonatine]

I'm really keen to start making side saddle aprons but getting a pattern is expensive (the one I've found is only available in America and the shipping is more than the pattern itself!). I do have an existing apron that fits me fairly well, so I'd like to try making a pattern from it. The issue is it's lovely and from the 1930s and I don't want to damage it at all, but aprons have lots of darts in to shape them around the knee and I can't work out how to copy the dart placement / size without opening them. The only information I can find online is for copying simpler things like t-shirts, does anyone know if there's a way to do this? Otherwise I'll go with my initial idea of trial and error with lots of mock ups!

[Ploshkin]

I had to Google side saddle apron as I had no idea what one was but I see what you mean about the darts.
I think i would do the pattern as a jigsaw puzzle.  Draw out the shape of each section between the darts and then all other flat sections so you end up with a load of shapes.  Lay the bits together with appropriate Vs between sections where darts are needed.  That should give you a rough shape for starters and you can add on seam allowances where pieces need to have a seam or tape pieces together where it can be one piece.
That's probably clear as mud but it works in my head.

[wrenkins]

OOooooo @Ploshkin. Clever! Use what you have got to work out what you haven't.
It seems so obvious when someone else says it. 

[StitchinTime]

Kenneth King used to have a class on Craftsy which showed how to copy a pair jeans, but I don't know if it has transferred to the new Bluprint.

I think he may have written something for Threads magazine, but I can't find the article I had in mind. However, I did find this one instead.

I think the technique is called "rub off" which might help with any searches.

[Sonatine]

Thanks @Ploshkin I see what you mean. I just got it out and had another look, the small darts look ok but there's a long dart (about 19") to run along the top of your right leg to the knee, where the dart then curves slightly so that the apron hangs down in the right shape. I can't visualise how it would open out without the dart in at all! There are also a few documents online with instructions to draft an apron pattern to your measurements, but they are generally older styles with a small gap in the skirt rather than a full apron. For example this one. Making one up in this style could be a step towards understanding the way the shape is achieved, even though it's not exactly what I'm looking for.

@StitchinTime I'll try searching that technique, thanks!

[Kwaaked]

@StitchinTime it's Jean-ius and it is on Blueprint.  They also have Next-Level Pattern Drafting From Ready-to-Wear which shows rubbing, too.

There is also the book Patternmaking for a Perfect Fit: Using the Rub-Off Technique to Re-Create and Redesign Your Favorite Fashions by Steffani Lincecum that I have at work on my shelf.  I used the latter Blueprint class and the book to copy a Chanel jacket and use the book as a refresher to make repros for one of the museums here when the garments are too fragile/damaged to use.

While these are aimed at garments, I have used the technique to copy seat covers for vintage/antique cars.  It's all the same, but if it's something you're not used to making, or out of the norm of your usual sewing, make copious notes.

Darts, I tend to mark where they were on the rub off and then split and add them after I am done.

However, this is an 1910 pattern online that might help visualize your pattern: here.  If you study it, you may even be able to copy this one.