First of all, power-clean all four hooves. Pick them all out.
Use the hoof knife to to scrape the sole and remove every last bit of dirt and flatten it out.
Next come the nippers. Depending on how long the horse's feet are, clip them according to how they should be, starting from one side to the other. Snip off the appropriate length and be sure to make it all even.
Last is the rasp. This is pretty similar to a nail file. Rub back and forth over the hoof wall and the sole to even out the trim and make it all smooth.
Now, I am not a farrier, so I am probably missing a few small details.