I once saw a YouTuber that said that to make your eyeshadow look less patchy that you must hold your blending brush such that you hand is farther away from the bristles. I tried this technique and my eyeshadows blended much better than before. So if you have patchy eyeshadow try not holding the brush too close to the bristles. Another thing that I do is use an eyeshadow primer and then lightly dust the eyeshadow primer with a loose setting powder, that way I have a smooth base for my eyeshadows. My go-to eyeshadow primer, is currently the Mac Soft Ochre paint pot, but I believe this paint pot more flattering for people with lighter skin types. There are seven different paint pots, so it is best to get one that will be flattering to your skin type. For example, the Groundwork and Indianwood paint pots might be best for people with darker skin types.
Once I have primed my eyes, I start out with a light amount of eyeshadow and then build up as I go along. Sometimes I use tape on the sides of my eyes if I want my eyeshadow to be more defined. To blend, use a clean blending brush and move your brush in circular motions. There are different types of eyeshadow brushes depending on how you want to do your eyeshadow. For example, a tapered blending brush can help to apply more concentrated eye shadow in the crease. A stamp brush is good for packing large amounts of eyeshadow onto the lid. A contoured blending brush can help to smoke out eyeshadows in the crease. An eyeshadow smudge brush can be used to smudge eyeshadows and diffuse harsh lines, and there are a good number of other eyeshadow brushes to give you the desire look you want.