Use 5-7% charcoal in your soil mix to improve drainage and decrease soil funkiness. This charcoal is very fine textured - size varies but it's the size of a grain of rice: approximately 1/2" long x 1/8" wide x 1/8" tall. Great for fine-textured roots (begonias, succulents, etc). 

Please adjust your recipe if necessary - they often call for 10-20% charcoal, and if you start with a coco coir base this will raise the pH of your mix too much. Charcoal is basic (high pH; about 9.6), coco is slightly acidic and perfect for plants (between 6-6.8), and peat moss is too acidic for plants at about 3.5. A coco-based mix with 5-7% charcoal is a perfect ratio.