If the leather is good vegetable tanned leather you can wet the loops and let them dry with cartridges in them. Don't try it with cheap chrome tanned leather. It will most likely get hard if you do. Oiling it will make it soft and floppy. The best way is to let them loosen up with use.
Most newbies to western leather push the cartridges all the way down to the case heads. Don't do that. Leave some showing at the top like these. Use your finger to push them up from the bottom instead of trying to pull them out from the top
