Use contact cement and follow the directions EXACTLY.
Or double faced tape for a less permanent solution. Clean the area where you want to put the patch with a little alcohol to make sure the adhesive will stick well. Don't do this if you are not sure as it might discolor that area. You can just use a little soap and water instead if you don't want to risk the discoloration.
Dry it well.
You can use superglue but it doesn't always adhere well.
You can use fabric glue for a less permanent solution as well. That's a lighter weight glue that comes off easily if you want to take the patch off later. Normally this won't harm most leather surfaces but it could. Use a blow dryer to warm up the glue before you take it off. The leather can be a different color where the patch was because it was exposed to less ultraviolet light.
Another way is to sew it onto the pocket. Whitten pockets open all the way so you can easily get to the area you need to sew.
Regarding decorating your case, it's YOUR case, do what you want with it. No one else paid for it, it's yours. Pros routinely stick patches all over their cases and they don't get knocked for it. I personally am not much of a sticker/patch person but I did put a bunch on one of my laptops. I put a Fury patch over the Acer logo and once I pulled my laptop out and one guy who saw it said to his friend that Fury was a good brand of laptops.
