Almond flour is the most important ingredient in your macarons. The brand and even how you store it can affect your macarons. Most importantly, you need to make sure your almond flour is very, very fine. If it is coarse, more like an almond meal, then even if you do everything perfectly your macarons will not be smooth and homogenous on top. You will see the mountain range of almond flour across the top.
If your almond flour is coarse, you want to put it in the food processor. When you do, add a little bit of powdered sugar with it and just pulse it. The powdered sugar will prevent the almond flour from turning into almond paste. Almonds are very oily so if you start the food processor and walk away, it will form a paste. Once you have your smooth almond flour, you still need to sift it with the powdered sugar in your recipe. In the grocery store, choose 10x over 6x. 10x has been sifted 10 times instead of 6. But of course, you can always sift your sugar more if it feels at all like it is clumping. You can take whole, blanched almonds and make your own flour in the food processor as well. Just remember that powdered sugar!
My favorite brands are Blue Diamond or Bob’s Red Mill (super fine when available). When storing your almond flour you want to keep it in as dry of a place as possible. Leaving it in the refrigerator or a dark cabinet will cause excess moisture which can affect your macaron batter. If you take a handful of almond flour in your palm and make a fist, if it clumps together into a ball then it may contain too much moisture. Spread the almond flour out on a sheet pan and let it dry out overnight. You can measure it ahead of time with your powdered sugar and set them out together. Then when you are ready to start baking, just sift and go!