Begin by using scissors to cut the ice cube sheet into strips that are approximately 10"×2" (for me, this was 1 cube by 5 cubes). Be very careful not to nick the water packs. After cutting, test each water pack for leaks by gently squeezing. If one of the packs leaks, the strip is unusable for this project (the remaining cubes of the strip can still be frozen and used as ice).
Once the strips are cut, measure the length and width, then place at least one of them in them in the freezer. Because of the tendency of the cubes to expand upward as they freeze, you will need the measurements of the frozen strips as well.