Just in case you have not seen it, here is some information on how to sew a tarp. It has a nice diagram for the flat-felled ridgeline seam. Well, actually, it is the central seam on this tarp, not the ridgeline.
It can be a bit daunting to get started on DIY sewing projects if you do too much research on the internet. Some people will tell you that your tarp will rip to pieces if you have the flat-felled seam running across instead of along the ridgeline. There are dissertations about how a mock flat felled seam is so much weaker and will not hold up in a storm. There are pages and pages dealing with the proper way of attaching tie-outs so they won't rip out. Since it's the internet, all these things are typically just opinions or speculation without any data to back them up. I have come to the conclusion that none of these things matter to a great degree. There are lots of people who make their tarps exactly the wrong way according to many things you read on the internet, and they seem to hold up fine. Ed Speers instructions are the perfect example.
I guess what I'm trying to say is don't stress out and get started. As long as you use some common sense, your project will turn out great.