The two pictures are the same keychain I did for my son who turned 16 (no new car, just his own key). One side is walnut with cherry inlay, the other side is cherry with walnut inlay. I used 1/8" thick cherry and walnut. I cut the inlay pocket about .080" deep in the .125" material for both sides. Cut out the inlay, glued in the inlay on both pieces and glued both halves together. Clamped overnight and then sanded the inlay smooth. Finished with mineral oil and wax to keep that soft wood feel in your hand.
It was all done in Easel using the inlay app.
… and simple. I may have to try that myself.