First, I had to find a pan big enough for marbling. I went to Tractor Supply and found a "droppings pan" made of plastic that measures 24"x24", and it's about 3 inches deep. This allows me to marble a piece of fabric that size, which is actually a pretty good-sized piece. (If you're going to try this for the first time, it might be easier to start with a smaller piece of fabric, which I learned the hard way!)
I mixed the methocel according to directions and poured it into my pan. The directions said it should set up firmly in 1/2 hour or so. While I was waiting, I cut some pieces of fabric into 24x24" pieces, soaked them in alum, and hung them on the clothesline to dry.
After over 2 hours of waiting, the methocel hadn't set up, so I took my blender out in the garage, set it on a stool, and scooped all the liquid out of the pan a few cups at a time and blended the heck out of it. That seemed to work, because it set up within an hour or so, and looked good.
This was my first attempt at marbling, and there are so many variables involved. The videos make it look SO easy...it really isn't, but it was a lot of fun.