I just got back yesterday from my first trip using my first DIY tarp.
I sewed a 12'6"x 10' SWT style tarp out of untreated brown ripstop nylon that I found for 25% off at the local fabric store. I made cat curves (parabolic curves for the technically inclined among us) on the long edges and left the short edges straight so they'd close off better. I also added tieouts 18" up from the edge on one side, hoping I could use them as ridgeline tieouts if I wanted to overlap tarps. Then using what I'd learned from warbonnetguy's "Make your own sil" thread, I waterproofed the whole thing.
For the waterproofing I used 1 tube of silicone and added enough mineral spirits of fill a 60 oz. juice bottle. I threw in a small handful of nuts/washers, and shook it up until it looked uniform throughout, and shook it a little more for good measure. I began by trying to just pour a little onto a sponge to apply the the tarp, but immediately realized I'd need a bucket to dip the sponge into. I worked quickly to sponge the mixture onto the tarp trying to get it a soaked as I could without running out. I applied the silicone to both sides of the fabric, then used a rubber squeegee to remove the excess. I left the tarp to dry and cure from Mon. night until Thurs. night before doing a garden hose test. It passed the hose test and since it was starting to rain I left the tarp up. I had some paper under the ridge and panel pull outs to check for drips.
All in all the tarp worked out great. There was fog all weekend and steady rain all of one evening and night. So I'm confident that it's waterproof. I did have one small pinhole leak on one of the panel tie-outs that I didn't catch in my testing, but I can fix that and it was a very insignificant leak. I used too large a needle for this job, which I'm sure didn't help anything, so I'll remedy that next time.
We packed up my OES 12x10 just in case my DIY catastrophically failed. It was good to have because we were able to baker hut the tarp to keep fire wood dry or sit under to escape the weather.
I'm glad I know now that I can make my own sil. I might purchase sil though depending on price and color preference. It's pretty awesome to have a tarp that pretty closely matches my coyote WBBB though.