You cut the vinyl tube into two short pieces, since they only need to cover the area under the printed coupler. There should be more, possibly much more (hard to remember 4+ years later) tubing than you need. I don't remember if they go over the motor shaft or the end of the Z screw (mine are long gone, replaced with what we call lovejoy couplers in the states, but are called plum couplings
from the eBay sources) but it'll be obvious which end they go over. Iirc they recommended that you soften them in hot water (dry them off/shake out the innards, rust never sleeps), but try them dry first. I think they were maybe 3/8 or half an inch, but check against your coupling (they only go to the halfway mark), and leave a hair (say 1/16"/2mm) sticking out.
They're actually a good method of providing a bit of flex between the motors and Z screws. Almost nothing ever lines up properly so that little bit of give will handle the little difference between the line (and center) of the motor shaft and the Z screws. Otherwise they'd want to jam.
The coupling clamps over the tubing and friction holds it all together. You don't need to get out that 4 foot (or one meter) cheater pipe to tighten it, just good enough it's snug and a little bit more. You want the gaps at the sides to be the same (mk1 eyeball is good enough, although I used a digital caliper to get them the same, overly excessive moose...).
There were some recommendations from those days to put a BB (as in BB gun, 0.177" metal ball) between the stepper shaft and Z screw, to give it something to pivot on. I bought a tube, but never installed them. I pulled the screw up a tiny bit so there was a gap between the shafts and no metal to metal contact. That gives the screw some space to move within the coupling without binding, and reduces the chance (and/or amount) of Z ribbing.
I used the tubing/plastic couplings for several years with good results, before upgrading the Z screws to TR8-8 screws and the fancy couplings. PB's method was pretty decent, no need to change until you've been using it a while and decide if you want to start on the mod route. At this late date I'd recommend keeping it pretty close to what you got, and if you outgrow it move to another machine. You can spend more time and money on modding this old design than it'll cost you to move up to the whizzbot 28 super printer...