Hi d600,
The brass jig is used as a gauge to hold the two sides of the ladder apart while you solder the strings ( sorry ex-fireman talk ) treads in place, this can be a bit fiddly as there is quite a bit of play in the jig, so use a bit of Blu-tac to hold it all together rather than singe the old fingers . Let us know if you need anymore help, this is what this forums for .
ATB,
Martyn.
P.S. The strings, treads or what ever you like to call them are just pieces of brass wire supplied in the kit.
P.P.S I did do a build thread on another forum and on looking back at it I did show how to build the ladders up, I'm not sure how to do a link but try this it seems to take you to the right place http://www.westernthunder.co.uk/index.php?threads/3-links-work-bench.1397/page-2#post-32459