Considering you don't want to use power tools, buy one of those cheap wood miter boxes. figure you're cutting on the 90 degree slots. Cut a new slot (on one side only - the side your cutting from) about an inch left of the 90 degree cut slot- allowing the strip to be pushed in.

Align it up for about a 7 to 1 scarf. Now screw in a block of wood inside so the strip can rest against it - not too thick so you can use a spring clamp to hold the strip to it. Maybe a 1 x 1 stock. You can use that to cut all you scarfs and they will match perfect. Get a back saw - with the spine that holds it stiff. It really doesn't have to be that supeer fine etc. You'll be able to cut scarfs like this in seconds.

If you have a work table it's faster if you can clamp the miter box in a vise and have your hands free.