The formula for the needed cord is (wrist size in inches) / (.17). It will give you measurement in inches. Sorry metricites. For example, if you are making one for a 6.5 wrist, then you'll take 6.5 / .17 = 38.23. So, you'll need a little over 38 inches of cord for "both" cords. If you are using 4 colors, then you'll need 19 inches of both color. Fused together, this will give you 38 inches. Yeah, I know. I'm a genius. Since I used 550 cord, this formula might be a little off if you are using 450, micro cord, or even 650 and above.
Tying this works best on a paracord jig of some sort, but it is not necessary. The jig allows you to keep the core tight as you braid the cord.
I have tried to include enough pics in order for you to see every step, but feel free to comment and ask questions if there is any confusion.