Not sure how widely known this technique is, but a friend suggested it... I got great results. This technique applies to plastic tarps. Here goes:

How to cut a custom ground cloth/tarp:
-I opted for a 5Mil thick tarp available at your local hardware store for ~ $15... the one in the photo is 8x10 ft.
-The tent I have is the TarpTent Moment (pic1, pic2).
-Trace out the shape you need (I placed the tent on the tarp, made some marks, then connected the dots using a straight edge... making sure everything was nice and symmetric).
-Now you are ready to cut (a flat smooth concrete garage floor is ideal for this task)...
-Use a standard soldering iron set pretty high (mine was set to 800F, nearly max setting)... place a straight edge (metal ruler OR similar) and trace your lines. Do a few practice cuts on scrap to get the hang of how fast you should go.
-Tracing with the iron cuts / melts the plastic tarp... preventing the fraying that would occur if simply cutting with a blade.
-Take your time and you'll have a nice custom shaped ground tarp. I was lucky enough to be able to utilize two of the grommets that came on the tarp.

Tarp as purchased:

Example cut with iron:

Tarp after cutting:

Tarp in (small) stuff sack (w/ 4 MSR groundhog stakes and guy-line):
(large stuff sack is tent; foot is for scale grin )