Check out this YouTube Video.

1 - Measure out the length of the double line you want to end up with. A 15-foot double will require 30 feet of line. Add another 2 feet to use as the tag end. The tag end is highlighted in green; the main line is highlighted in red.

2 - Loop tag end over main line, and back under the tag end. You now have three ends to work with.

3 - Moving the main line to the left, go over the first line, then under the tag end.

4 - Continue the plait by bringing the top outside line through the centerlines by going over the first line, then under the next as illustrated. 1.5" length on most line classes is sufficient.

5 - It is important that the plait is tight. Pressure exerted by pulling the lines down tightens it to some extent. It is easier to tighten it by simply jerking the ends up as shown in yellow. Until you get used to doing the plait, don't worry too much about how tight it is.

6 - To end the plait, form a loop in the tag end and continue to plait for another three cycles. Important: Form the loop when the tag end is in the middle as shown.

7 - Feed the whole double through the tag end loop as shown.

8 - Pull the tag end up till the loop snugs up. Moisten it before you pull it up to lubricate the ends. Do not pull too tight, as you'll bunch the plait up.

9 - Trim the tag end close to the plait as shown. Practice the plait until you are confident in its strength, and get it as tight as possible. Once you're happy and confident with it, USE IT!

Reproduction courtesy of Peter Pakula