If you have access to a laser cutter, you might be able to get it done locally (assuming you can source the leather). It's smelly and will leave slightly darkened edges (I recommend using masking tape all over the side that's being cut, to protect the leather from charring), but it's fast and precise. Alternately, if you have patience, using an X-Acto knife will do the job just fine on thin leather.

I got the applique laser-cut from chevre (goat leather) for these shoes I made, to give you an idea of what's possible: https://www.dropbox.com/s/t8x3w4dwrpz7fdq/2018-01-19%2013.18.51.jpg?dl=0

If you plan to glue them to your existing leather boots, here's some tips:

• Sand the surface of the boots where you plan to adhere the leather
• Paint both sides with permanent solvent-based contact cement like Weldwood (which should be available at most hardware stores, at least here in the US) or Barge. Sewing is nice but not actually necessary; contact cement is pretty tenacious. If your existing boots aren't leather, I'm not entirely sure how well it will stick, but it's worth a shot. DO NOT use hot glue; you can get Weldwood for cheap (https://www.amazon.com/00271-Weldwood-Original-Contact-Cement/dp/B0006MXRY8, or a small bottle with brush https://www.amazon.com/107-3-Ounce-Weldwood-Contact-Cement/dp/B000M2SKNU). Just be prepared to sacrifice the brush you use.
• Let both sides dry, stick them together, and then hammer to make it adhere firmly
• If the base boot is NOT suede or nubuck, you don't have to worry about getting the glue precise; you can rub excess off, just like with rubber cement.