1 Sling Harness
Simple and easy to make, however can slip down if theres not a continuous pull upwards making it ineffective. If you have a small waist like mine a 120cm will work perfectly. To make pass the sling around your back and bring both ends to the front. While holding these reach between your legs and grab the lower loop, bring this between your legs and clip everything together with a screwgate 'biner.
If you have a slightly wider waist use a bigger sling, but when you pull the loop through your legs tie an isolating knot, (overhand will do), to make it tight and shove the excess away down the side of the waist loops. Because this has to be weighted upwards to work it only really works for abseiling or seconding.
This one works much better than the other one and a friend has actually used this one when he's forgot his harness. To make take one 120 sling and find the centre, to do this double up and shuffle the two ends until the sown part is in the middle. Tie a overhand each side of the sown part and try on your legs. I find that tying another overhand on each side makes it fit my legs perfectly.
Next take the other 120 sling and pass it around your back above your hip bones, as you would with a normal harness. Tie both ends together with half a reef knot, passing one end through to other to finish it off. This allows you to pull it tighter and still easy to remove. Then tie another overhand to make a loop. Pass a 'biner through this loop and the leg loop and there you go a harness. It's not the most comfortable thing in the world and you wouldn't really want to fall on it!
Both of these would be more comfortable with thicker slings, I just used what I had to hand. I would not suggest falling off wearing one of these but if you had nothing else to hand they'll work.
You'll never have to abandon a day's climbing again!