The way I string tarps is with a ridge line, prussics and two small carabiners. What follows is my preferred way to setup a rectangular tarp in a basic A-frame roof style.
I start by setting up a ridge line between two trees using a siberian hitch knot (aka an evank knot) on the first tree. It's a quick-release knot that's easy to tie at arms length on tip-toes when you want the ridge line to be high. The free end of the ridge line is attached to the other tree using a trucker's hitch so I can re-tighten the line later on, as even a very static line will stretch and sag a bit over time, especially when the line and tarp are wet and wind blows against the tarp.
Next, I attach the tarp itself to the ridge line by
suspending it
beneath the ridge line. I do this because it's fast, easy to do in a strong wind, and prevents wear and tear on the tarp where it would rub against the ridge line if it were draped over the ridge line.
The tarp is suspended from 3 points: the exact center tarp loop and two tarp loops at the middle edges. Here's a link to a brief video clip showing how fast and easy it is to suspend a tarp this way:
http://s1122.photobucket.com/albums...inging/?action=view¤t=Tarpstringing.mp4
The center loop is suspended from the ridge line with a short bit of cord. I just pass the cord through the center tarp loop a couple of times, then wrap the two ends around the ridge line two or three times, then tie the loose ends in a bow like a shoe lace. This won't move in a wind, but you can slide this knot along the ridge line to reposition the tarp along the line. This bit of cord stays permanently attached to my tarp so I never need to look for it.
I use proper prussic knots at the two middle edge loops with two short loops of cord. To make for a faster setup, I wrap my prussic loops 3 or 4 times around the ridge line and instead of tying these directly to the middle edge tarp loops, I attach them with little carabiners. These carabiners and little prussic loops also stay permanently on those middle edge tarp loops. Like the center knot, these two prussic knots won't budge in a wind, but they can be slid by hand along the tarp to reposition the tarp or make it more taut when the tarp stretches and sags.
Because these middle edge loops are attached to the prussic loops with carabiners, you can tie a light cord from one carabiner to the other UNDER your tarp to serve as a clothesline if you wish.
The carabiners and prussic loops remain attached to the middle center tarp loops after I take down my tarp so they will be ready to use the next time I string my tarp.
Next I guy out the four corner tarp loops to whatever trees are suitably located for this or to ground stakes, roots, or heavy rocks if necessary. I attach my guy lines to the corner tarp loops using a rolling hitch so I can lengthen and shorten these guy lines as needed without having to untie and retie them from their anchor points. I then make a small loop or bight in each of these 4 corner guy lines about 3-5 feet or so from the corners of the tarp so I can later attach my middle lines to my corner guy lines. The free ends of the guy lines are attached to whatever anchors are available, usually with a quick-release siberian hitch.
The remaining tarp loops located along the edges of the tarp are then secured using short lengths of cord (4' to 6') to the bight loops I put in the main guy lines. So, these
secondary guy lines are actually attached to whichever guy line is closest to them. These are also attached to the tarp loops with rolling hitches and the free ends are tied to the bights of the guy lines with any simple quick release knot (usually the siberian hitch again) These secondary guy lines are also tied using rolling hitches, which makes for a nice taut tarp.
Because my rectangular tarp is sil nylon and sheds water in part by being drum tight, I'm pretty fussy about a taut tarp. I also own a very big wing tarp with catenary curved edges, but that requires a slightly different setup.
I hope this description makes some sense. Below is a pic of my wife stringing our rectangular sil nylon tarp.
Hope this helps,
- Martin