http://knittyblog.com/2017/08/3-tips-for-drumcarding-top/
There are a lot of resources out there for learning how best to use a drum carder, these past three articles are a great resource for introducing a lot of the concepts involved in drum carding. This blog is probably a good one to follow also.
With this article, I especially liked the tip where the author says to hand card some of your smaller bits of fiber first to spread it out a bit more. I had never thought of that before, but it makes perfect sense to keep things thin and even.
Happy Crafting!