Fitting single layer shell cordovan straps used to drive me nuts–until I figured out this trick. All you need are some round wooden toothpicks, a tool to clip the toothpick, and a spring bar tool. Diagonal cutting pliers or wire snips work best to clip the toothpick.
Basically the issue is that the strap end is not tight around the spring bar, which makes getting the bars into the lugs a real pain. By inserting a toothpick "behind" the spring bar, we can stop it from wiggling around in the strap-end.
Step one is to insert the spring bar into the strap and then put the toothpick in "behind it" (farther away from the lugs):
Then you push the toothpick all the way through behind the bar:
After that, clip the toothpick on the longer side:
Then use your spring bar tool to push the the toothpick into the band so the cut end is about 2 mm inside the band (not shown). Then clip off the other end:
After that, use the spring bar tool to push the toothpick in so it is entirely inside the strap end. You now have a strap that is very easy to install!