Make sure you extract them correctly, that is the number one problem people have. They need to end up in the "custom scenery" folder, and the folder holding the scenery must not be in a sub-folder of itself. Look at some of the airports that are in the "custom scenery" folder to get a feel for the correct hierarchy.
Othern than that, there is really nothing to it, but some sceneries (most on the .org, actually), require you to download extensive "third-party libraries" like OpenSceneryX, which also must be installed to the "custom scenery" folder.
Good luck, Jan