Yes, you will need to use mortar to stick these, probably would want to use a polymer modified mortar, it’s are a bit like a tile thinset. Don’t forget a trowel, 😉 These are cultured stone, but they do not come on a sheet. It will come in a box, ranging from like 8-13 square feet. You will need to buy a whole box, most likely. Get the old stuff off the scratch coat as best you can off wall. Then make like a suction cup on the back of your stone by making a rim around it with your mortar. Some guys will just plop a bit on the back and furrow a channel. I wouldn’t recommend this unless you have a boss breathing down your neck. Press if firmly and quickly (not hard and fast, if that makes sense) into the wall. Hold it for like 3 or 5 seconds. Done.