Identify the hole. One easy way is to pump the board slightly then use soap and a sponge to wipe the board. Where the bubble forms is your hole or slice.
If soap and a sponge doesn't initially work, pump up your board to the recommended psi and try again.
Clean the area surrounding the hole very well. Get it as dry and clean as possible; you can use rubbing alcohol. Then, circle the hole (or, in this case the slice).
Now that you have identified the hole, deflate your board. That way, when you later glue on the patch the air in your board won't cause it to pop off. Once deflated clean the area around the hole.
Take out your repair kit. A good manufacturer will send you a patch for at least 3 years after your purchase. If the glue is new you'll punch it open with the pointed underside of the cap.
Cut a piece of the material with enough coverage on all sides of the hole or slice. Round the edges of the patch.
Trace or outline the patch with a pretty pencil. Make sure the patch is centered on the hole. Do this so you don't spread too much glue in the next step.
Glue one side of the patch (it doesn't matter which side). Glue up to the edges with a thin layer. Then, glue inside your marked area all the way to the edge. The glue should cover the entire area.
You can use the end of the tube of glue to spread the glue, squeezing out a bit at a time. A toothpick or Popsicle stick is probably a better idea.
After you have glued both sides well, let them dry for 6 minutes before putting them together. Again, let the glue dry a bit before putting the patch on the hole.
Use a spoon or smooth surface with an edge to push all the air bubbles out from under the patch. Apply pressure so that a tiny bit (or none at all) of glue comes out of the edges.
Let your patch dry overnight. Twelve hours is good, 24 is better.
24 hours later, inflate and go SUP. If you are nervous that you didn't do a good job, inflate and wait another 24 hours to see if any air escapes.