The most important thing I think is to have no holes in the cardboard and to overlap the seams well - at least 6", so seedlings that sprout underneath have no way up and out. Remove anything in the area that could poke holes in the cardboard, and any tape or staples on the cardboard. If you have access to a tiller, tilling the area first, then raking it reasonably smooth, would be ideal before laying the cardboard and then whatever organic matter.
If you're working in a spot with vine grass like Bermuda, St. Augustine, zoysia, etc... Going around the perimeter of your proposed garden area with your shovel, plunging straight down, will be worthwhile to make sure no grass under the smother is being kept alive by being connected to grass outside of the smother.
