Prepare the leather parts, cut out the shape you want and metal fittings.
In this page, we will use LeatherCraft.jp's kit product "Inlay Keychain A1 Kit".
For the metal fittings, a Double Split Key Ring 25 mm is ready for this kit.

Skive the edge of the core leather using a French Edger or Skiving knife.

Skive the edge of the fresh side of the main leather part's window using an Edge Tool.

Finish the edge and flesh side, which of the main leather part's window and become a loop part when it will be folding back later.

Attach the core leather to the main leather part with leather glue.
Adjust the position to exactly match the window.

Attach the inlay leather to the main leather part with leather glue.

Pass the Double Split Key Ring through the leather part.

Fold back the main leather and glue them together.

Finish the leather edge and sew around it.

Completed.