I have a big three-ring binder with poly document sleeves in it that I use to keep “studies”, mostly scraps of leather with difference thread and stitch combinations. I’ll tie an index card to each with the “recipe”, or just throw it in the poly sleeve with the work. That way I can flip through it like a sample book and replicate what I see.
My favorite stitch I’ve done so far was on the quarter seams of my fourth pair of boots. Alternating black and white saddle stitch, with every third mark skipped, repeated in three staggered rows: