The link posted above is an excellent pattern that a lot of people (including myself) have used. I highly recommend it.
silnylon is not absolutely required. Other waterproof materials will suffice but with a weight penalty. If this is your first tarp project I would suggest you proceed with the project. Best to make errors on Joanne ripstop than more expensive sil. If you don't like the results you can always cut it up for stuff sacks later.