Are you talking about making them into a survival type knot like the bracelets?
That shows how to make bracelet. You would just need to adapt it to a longer length for your reins. The only problem is once you tie them up the can be pretty stiff. Not sure they would be great for reins.
For a cobra braid like the survival bracelets, you need 1 foot of paracord for every 1 inch of completed braid.
For a round braid, I couldn't say exact amounts but I would probably double the amount of cord for the length I wanted to ensure adequate amounts. Paracord really is cheap, and (at least here) purchased in 100ft/30m bundles normally.
I would not use an intricate braid like cobra on reins. Instead, think about whether you prefer flat or round reins. You can do a round braid in any multiple of four (4, 8, 12, 16 and so on), and the more strands the thicker your reins. With round braid, I would recommend using (2 lengths = 4 strand round braid / 4 lengths = 8 strand round braid) strands that are TWICE the length of your required amount (for 8ft reins, roughly 16ft per strand so cut 2 or 4 32ft lengths), put the strands though your clip to their centre (so the strand is doubled, being held by the clip in the middle), and braid like there's no tomorrow.
The important thing to remember is that paracord is hardy and unless you cut it too short, you can always undo your braid and try something different if your first choice doesn't feel right as reins.
For round/flat braid instructions, check out this site. Personally I would probably go with an 8 strand (4 lengths) round or the 6 strand (3 lengths) flat braid for reins.
ACTUALLY. I may have exaggerated the lengths a little. For an 8 strand round, I would probably cut ONE 100ft bundle of paracord into quarters, and the 25ft should braid into more than 8ft total. But don't quote me on that. I've always had trouble finding exact numbers on cord usage in braiding. Just remember the thicker your braid, the less completed braid you'll get per ft of cord.