I am not sure how to simplify that.
Just open up your EU game directory. Check out the what KIND of files go where, in general. In /scenarios, if the files are like Grand_Campaign.eug and 1492.inc, and in the zipped homemade scenario file are files like MyCoolScenario.inc and ASetOfCountriesToPlayInIt.eug, then move those files into your /scenarios subdirectory.
Some of the files have to overwrite the current files, at your risk, of course. If the scenario has a new colonists.csv file, you ought to find where your current colonists.csv file is, make a backup so as not to screw yourself, copy the new file over the old one. Tada!
Each zipped file is different, since some people are content only to modify a few files, and others want to do a near-total conversion of the game, like the IGC.