My friend and I have been making our own modified English Civil War style baldrics for about a year now. Ours are 3 part baldrics. We've sold some but mostly given them away at our events. We're looking at having a jeweler make some PC buckles but they'll be pretty expensive.
We do it this way.
1) Make your frog. Figure out the angle of your sword-from straight back to steep angle. This will determine where the front strap will be mounted.
2) Measure(w/seamstress tape) from left belt line-over the right shoulder- back down to left belt line.(base length) Add however many inches till it sits at the hight you want.( this is the overall length). *if you are NOT using a buckle ie:single strap-skip steps 3 + 4.
3) Figure out where you want your buckle. This will give you the length from frog front to buckle. Add 2 inches, 1" for mounting buckle, 1" for mounting to frog. Mount your buckle onto the front strap.
4) Figure out how much of the tongue you want past the buckle, add that to the length from the frog top.(back strap). Add how ever many inches you will need to attach to top of frog. (back strap).
5) Attach straps to frog.
This is a VERY basic way to do it, there are a few adjustments here and there. I suggest doing a test run on cardboard/construction paper. I hope this helps and stirs the creativity. I am open to any suggestions.