Begin by making a simple pair of plain reins out of prepared 1/8' or 3/32" kangaroo lace. These reins should attach to the bit at one end and buckle together at the other. Most laces retain a bit of curve even after they've been prepared. Use this feature to your advantage by always making sure the curve faces down.
Position your assembled reins on the model and mark the area where you'd like the lacing to begin.
Using your smallest (03) pencil punch, make a hole in the marked area.
Continue punching holes down the entire length of the laced section.
The holes should be approximately 1/8" apart and placed right in the middle of the strap. If you let them get off center, you will be far more likely to break the rein while lacing it.Real reins are laced with a narrow leather strip and there are some model tackmakers who do this as well. I admire the effort but so far haven't felt compelled to duplicate it. Instead, I use waxed linen cotton thread which can be purchased very cheaply (less than $5) at Hobby Lobby.
Prepare the thread by cutting a piece approximately four times the length of the rein's hole punched section. Separate the thread into four strands and run each of those between your fingernails to remove the excess wax. Take two of the strands, lay them side by side and squish them together making sure they stay flat and do not twist over one another.Cut one end into a point and thread it through the eye of your needle. Use the smallest needle possible. A good rule of thumb is this--if your needle can't pass easily through the holes in your rein, it's too big. Starting at the bit end of the rein, push the needle through the backside of the first hole. Pull approximately half the length of thread through the hole, then pass the needle under the rein and back through the front side of the first hole.
Gently continue pulling the thread through the hole until the first loop is complete.
Continue passing the needle though the holes down one side of the rein. Be sure to keep an even tension on all the loops and try not to let your threads twist over one another. They will look better if you keep them lying flat and parallel.
As you work your way down the rein you will probably notice a corkscrew shape developing. Do not worry about this. It's perfectly normal and will go away once you lace the other side.
When you reach the last hole, slide the needle off the thread. Attach it to the long piece of thread still sticking out of the first hole. Repeat the lacing process down the opposite side of the rein.
You should now have a nice length of evenly spaced sideways "V's." If yours are loose or uneven, you can carefully go back with an awl and make some adjustments.
When you're satisfied with the lacing, flatten the loose ends onto themselves, snip them off with a pair of sharp scissors...