Door Casing Gap Against Cement Board
The shims come ready made at lumber and big box stores or if you have one you can make them using a miter box and a 2X10 or the like.
My miterbox is an old Hitachi slide compound. It can cut a piece of material about 12" wide allthe way through. When I need to make shims I cut off scrap 2X10 or 2X8 blocks about 12" long. Turn the blocks 90 degree so the end cut is up against the fence and set my blade at .5 degree. I cut the edge to start one side, now you have a block that is slightly narrower on one side than the other. Then flip the block so the edge that was facing me is now up against the fence. I move the block so the blade is at the edge of the cut that was made first, but I line it up at the block that has the most narrow side, so when I cut the block again, the shim is made. I can be but not necessarily sharp on one end and about 1/4-3/8" thick on the other.

Keep flipping the block until the part you hold onto get too small, and start with another block. Needless to say you must be mindful of what is going on, too close and you could lose something.