Our tarp is 20 x 24 and we use poles in every second eyelet and a ridge pole in the middle. It's been through a lot of heavy rain and strong wind and we've never had a problem. The trick is to keep everything tight, use long pegs, double ropes on the corners, heavy duty springs and ropes with metal tensioners. We even throw a rope over the middle for extra protection.
Gypsylady recommended wooden tensioners but we've been in conditions so bad that the wooden ones broke. It's metal all the way now!
As for set up, lay the tarp on the ground and peg out your corners and middle. Pole and rope each corner side starting at one side then the other. Next pole and ridge pole the middle. You'll now have a basic A-frame and can work your way around pegging and adding poles.
Just a bit of warning though - the poles for a large tarp will weight a lot. If your budget extends a bit, look at aluminum poles. They're $15 each
but lighter and stronger. We'll hopefully replace ours with them slowly.
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