“…outlining the tent onto the tarp using a marker, and cutting the tarp on the marker lines. Would this be sufficient?”
You would want your ground cloth smaller than the tent footprint. Otherwise, a large cloth will catch the rain and funnel it under your tent; and your body pressure would eventually make it leak through.
As people noted, you might not need a footprint. But a footprint has saved my tent from:
1. Staining. There are several places whose dirt will stain your tent.
2. Holes. At night, when I place my tent down, I sometimes don’t see that small root jutting up. The ground cloth gets punctured, but the tent floor gets saved.
Some material options:
1. To save weight, try 0.7mm painter’s cloth. I’ve used this a few times.
2. Tyvek dries fastest in the morning. My other plastic tarps catch a lot of ground dew and take a while to dry (in humid areas of course).
3. For super ultralight use window insulation like
http://www.amazon.com/3M-Company-2141-Indoor-Insulation/dp/B00002NCJI . This appears to be the same material as the GG cloth
http://www.gossamergear.com/cgi-bin/gossamergear/polycryo_ground_cloth.html at only 1.5oz for a 2-man tent. This takes a little while to dry in the morning with the ground dew capturing all sorts of goodies on the ground cloth. But I use this option the most since it’s the lightest and surprisingly strong.
I also think of the ground cloth as dual use since I can lay it down for a lunch stop and lay on it. This prevents my pants from soaking up the area’s moisture.
-Barry