Most of us use an edge guide like this one when we want to make a cut an even distance from the edge of a piece of leather, like cutting the border for a holster.
These guides work great except for when you need to follow around a tight inside radius, like the cutout for a trigger guard. I modified mine and it works great in those tight places. This photo shows the way I filed the guide to fit an inside radius. This works well down to about 1/4" radius.
It works great although along straight edges, it doesn't even out any irregularities the way an unmodified one does so you have to take care to not let it drag on the edge of the leather.