Be sure to line up the top edge of the ankle cuff with the sweater ribbing, so your slippers will have a nice edge. It will also enable them to stretch well over your heel when putting them on.
Pin or put weights on the pattern pieces and cut them out. I usually cut through the 2 layers at once so I end up with mirror images of my pattern. Just be careful that a seam placement or knit pattern on the opposite side does not differ from the side you are cutting on. I also make sure that the upper piece and sole are not on the ribbing part of the sweater. Ribbing is usually wavy and it does not felt like the rest of the sweater.
Another option, depending on the thickness of wool, is to make a double soled slipper for extra cushion. This is a little more difficult to sew, so if you are not really experienced, you may want to skip it on your first pair. The sweater I choose was kind of thin, so I choose to double up the sole.
You can also hand sew a leather sole on for added protection. My daughter just starts to wear out the sole of hers before she needs a new size, so I have never done this. It would be a good idea for feet that have stopped growing though. I think it would really extend the life of the slippers.
The following picture shows how I placed my pattern pieces and cut them out. (Do you like my excessive pattern weights? Hey... those patterns were going nowhere!)
(Click the icon on the picture to enlarge for better viewing.)