My horse has a few chronic spots in his shoulders that I work out regularly. It's kind hard to describe. It might help to understand what you're working out. What the knot is, are muscle fibers that have clumped together and are blocking blood flow. When you massage them, you are allowing the clumped fibers to relax and separate which allows the blood to continue to flow.
When you find the knot, start with the most hand surface area possible. The more finely you touch the knot, the more pronounced it is to your horse. Just use the bottom of you palm and knead the knot like you were kneading dough. (I know, nobody in the 21st century kneads dough. It's the only analogy I can think of). Just use a gentle, rhythmic kneading motion. Once you feel it start to work apart, switch to the outside edge of your hand and run it up over the area to encourage the fibers to stretch out. Your horse will tell you how much pressure he's comfortable with. Initially he probably will resist, but then start leaning into you once it starts to break apart. Be aware of where his rear legs and mouth are the first few times you try it!
You just have to see your distance...you don't have to like it.