Alwyn, I've tied a few of these in the past, they're fiddly.....first sort out the treble hook mount ( I use a light pike wire looped behind the eye, enough movement without being droopy, if you get the 15lb nylon coated stuff - drennan I think) then tie in a length of Mylar over the pike wire to just a bit more than you want, measure by offering up the front hook ( obviously the pike wire should be about 5 inches to make it easy to handle) so the pike wire protrudes ( two stands obviously).
Then take your front hook and point first thread it down into the Mylar, ignore the pike wire to start with, let it protrude out of the way. Inch worm the Mylar down the hook until you've about the length of the shank ( it's not that much really maybe 3/4 inch) and let the hook point come through the Mylar. Then feed it round the bend until it's on the top of the shank. Mount the front hook in the vice, thrown a couple of turns of thread over the pike wire strands to hold it, then ease back the Mylar back towards the bend ( like a sock) then I clamp a hackle plier onto the hook shank to stop it springing back.
Next undo your holding turns of thread and get the pike wires on top of the shank, line up the treble as you want it ( some like two points down and one up like a keel) and bind down the wires, then double them back over and bind down again. Coat with super glue and give it a minute to dry. Remove the hackle pliers and spread the Mylar back along the shank, if you've brought the tread back to the head it's simply a matter of binding down the Mylar and putting in your throat, wind etc....
I hope this helps, I used to tie these, but have long since given them up as they're far too labour intensive to tie. Irrespective of what people saw snakes don't whip around like sandeels in the river, they just fish in a straight line like a tube, the treble will sometimes catch in the Mylar when casting rendering them ineffective till you discover it, when you do get a fish they write off the Mylar body ( once you've had three fish they're knackered) if you hit a tree or a snag that's 20 minutes work gone rather than three minutes with a tube. If you want to mimic a slim sandeel, just tie a needle tube with no body a small throat hackle and a longish wing with some laterally tied in mirror flash longer than the wing as 'lateral lines' it's far more effective.