Every year some poor bugger who has probably never done it before it gets to rig the tarps and be responsible for any soggy miserable nights that might result if it's not done well enough. Starting from the bare space it's really quite hard to know where to start if you've not done it before, so this doc attempts to provide some advice.
Until 2014 we had a shortish green tarp (10mx6m) wrapped up the back edge at upper entrance and a separate tarp for that upper area. It wasn't very good. In 2015 we bought a much longer big white tarp (15x6m) which covered the whole rear area down to the ground. In 2024 we bought a replacement but due to a documentation error that one is 15x10m which is really too wide. This description has been updated for the 2024-on version.
There are two essentially-separate jobs:
If there is any chance of it raining soon, start with the water tarp, as you really don't want to miss collecting all that water, and setting it up in the rain is rubbish.
I will describe things as if standing at the bottom entrance looking up, because that works for both sections. So the left wall is the gear/stove/animal-hole wall. The 'back' is the top end of the bridge. The 'front' is the main entrance.
Tie bungee-balls (not rocks!) into the tarp to hold it in place when you do not have eyelets to hand. Rocks are pointy and damage the tarp. We have plenty of bungee-balls - find them.
Since 2015 we have had one really big tarp. This covers the whole sleeping area from front to back. It is set up so that essentially the whole bivvy apart from the water-tarp area is covered. The 2015 tarp was just about wide enough to span the width of the bivvy (i.e. the right size (15x6m)). The 2024 tarp is 3-4m wider than the bivvy, so has to be folded along the long axis to work. It is 15mx10m.
Get out the (heavy!) big white tarp. The long edge goes from front to back of the bivi. The basic position is that it goes all the way to the right wall, partly 'underneath', and the left hand edge crosses the steps up the middle, covering the whole width at the back. The back edge reaches the floor behind the top 3-person flat area at the back entrance. The front edge comes down to ground level. Someone may have written 'front' and 'back' on the tarp to initially orient it.
So start by opening it out to full length over the sleeping areas to the right of the steps, and folding 3.5m of the right hand edge back over on top. This needs to be reasonably neat and square, as you can't get to it later. Then pull the folded RH edge close to the wall. Sequencing the ropes is a bit tricky: you need the tarp vaguely in place, then install the four main ropes to lift it up. Then finalise the tarp position and tension it.
Once the tarp is somewhat spread out, get the 4 main ropes in place underneath it. Use the anchors shown on the diagram that are known to work. The secret to the central rope '2' is _not_ to use the obvious ceiling bolt which is hard to reach; instead tie the RH end of rope 2 to a tension rope '5' above the steps. This allows the tarp edge to come further left and run square to avoid sagging. Tension rope 2 (with rope 5) first, then 1,3,4 to lift the tarp up and ensure it is neatly spread. You will need bungee-balls to hold the RH edge of the (folded) tarp in place. To get good tension on the main lines thread them through a maillon, and tie off on another bolt. You don't need to add any new ones - there are more than enough bolts now.
At the left-hand end of rope 4 the tarp gets scrunched round the anchor. This is fiddly to get it all to hang nicely. The objective is to get the whole tarp taut enough and square enough that almost all the water runs down the top and out the front, rather than collecting in 'buckets', which then drip. That means that it has to be fitted quite 'flat'. To make this theory actually work, add extra lines in a zig-zag pattern between the main ropes so no buckets can form. Don't tension the early ones too hard as they distort the main ropes - it's a balancing act to get the tension everywhere right.
To keep the electrics dry the edge of the tarp is held taut with string to concrete screws in this area.
At the front edge the tarp lays over rocks. Put some padding (karrimat/old tarps) over the rocks to avoid damage to the new one. Lay rocks on the edge on the RHS. On the LHS use string to tie the front edge down.
This is the hardest part to get right. A couple of judicious bungees tied up high at the back wall will stop 'buckets'. [More detail should be added here]
Use a long/thin (2x5m) waterproof 'logpile' tarp. It is hung between two side longitudinal ropes, with lots of bits of string attaching the eyelets to the side ropes. Aim to hang it under the hole in the roof, and far enough forward that water off the front falls onto the 'water platform' (not into the food boxes). We now put two of these up to avoid drips and add water-collecting redundancy.
It's easiest to approximately tie the tarp to the side ropes with adjustable knots, then put the ropes in place, adjusting as you go. The outer line ends up too high to adjust once properly in place. Adjust it to catch as much of the water falling through the hole as possible, and slope gently, but consistently downwards.
The funnel/filter is hung below the end of the tarp where it will get nearly all the water and is much easier to hang/adjust/unclog.