Just double the length of your strings and start braiding in the middle. Say you're going to use 35' per string to braid a 23-24' mecate, make each string 70' and loop in the middle over a nail or whatever in your vice and start braiding. The easiest is to find the middle on all your strings and tie together in a loop to keep straight. Then tie each in a tamale and wrap with a good rubber band. When you get that done, untie the loop and start. Make it good and tight at the start. When done just go back and unloop and put in a nice piece of leather for a popper. You can also do that at the start. The finished end just tie a turkshead. If it's a 12 strand rope, tie a 12 strand knot, then tie an 8 strand so it has a little taper, keeping all your ends out the end. If you've done any button work you could stay with just a 12 strand knot and leave your ends stick out and tie a pineapple knot over the tied knot.