Tied the same way as using a single strand for a paracord bracelet , but using two. You don't wrap the cord twice around the buckle ends, like the single strand on a 1/2" or 5/8" buckle, since there's now two center cords looped onto one end.
Measure out the wrist length with the four working strands wrapped around the other buckle end and pair them off to start knotting.
I used around 16 feet of paracord(finished amount used) for the doubled one, for an 8" wrist. I most always use more than I need, starting off with two 12 foot strands, and ending up trimming off 2 feet of each strand at the last knot. It doesn't go to waste since I'll use the remainders for zipper pulls, paracord cross, etc...
*Added a few pics for those that had a bit of trouble visualizing the start. You find the center of the two lengths of cord and use a lark's head/lanyard loop/cow hitch, to attach them to one side of the side release buckle. I've shown two ways to start, either works fine: side-by-side or one in the center of the other.
Then run the four strands around the other buckle end, for the wrist size(I usually add 1/2" to 1" to the actual wrist measurement for a comfortable fit).
Laid next to a ruler/tape measure, you're measuring from the flat side of the female end of the buckle, to the base of the prongs of the male end of the cord. You don't figure in the prongs, since they'll be inside the other part of the buckle when worn. As shown, I'm measuring it at about 9" to fit a 8.5" wrist.
You'll start tying the knots from there. After you tie each knot, hold the core strands with one hand and push the knot up towards the buckle, to tighten them up and work any slack out as you go.
When you reach the other end and can't fit any more knots, you can finish by either using something like hemostats to pull and tuck each of the ends thru the last knot, then trim and tuck the ends in, or trim and sew/melt/glue the ends to finish.