Cutting leather: piece of leather, sharp knife, 500mm steel ruler, square, pencil, clamps, cutting board (I use two from Ikea side by side)
This is the moment of truth, screw up and start all over again until lines are straight, risking to transform planned 75mm into a 50mm strop before it is all over. So clamp it, clamp it good, just holding on to steel ruler and leather by hand is asking for problems, ruler or leather or both will most likely move and throw knife off the straight line. Otherwise it is a pretty straight forward affair but adding a hold on the cut off part of leather using fingers the last couple of inches prevents slipping at the finish line.
edit: got two knives, a Stanley and a cheap copy called Sheffield Made in China, I hold the Chinese as my favourite as it is steady with minimal blade rattle.
edit: checking steel ruler for burrs is good practice to prevent ugly marks on leather.