This step is one of those things that takes ten seconds to learn, but a lot more time to actually do. It is not difficult, but it can be a bit messy.
Snip along all the seams up to the sewing, but be careful not to cut the seams. Keep doing this about every 1/2″ (1 cm) or so along. It is not an exact science, so there is no need to use a ruler or anything – just eyeball it and close enough will do. This does get hard on the hands after a while, so be sure to take lots of breaks. Spring-loaded scissors help too if you have them. It might also leave little bits of thread everywhere, so be ready with a broom or vacuum.
This is probably the most difficult step, so kids may need help. When this is finished, you are almost done!