How much fleece you need is going to depend on the size of your kid! The length is up to you, and if you want it to wrap around their feet while they're on the couch, or if it should be shorter for them to run and play in. Once you've determined that length, add the length you want for the sleeves, and about 9" (23cm) for the cowl.
These particular snuggies used 2 yds (1.8m) of 60" (152cm) wide fleece each. Any width of fleece will work - it just affects how much the finished snuggie will wrap around the child.
You'll also need a measuring device, some chalk or disappearing ink pen, a pair of scissors and some thread.
I decided to use contrasting thread and the Singer serger to flatlock the seams together (more on that in Step 4!). You could use matching thread and a sewing machine, or sew by hand. The choice is yours!