First, check to see if it isn't already in their layer. Otherwise people tend to yell at you.
Second, Make your placemark. Top of the screen, the yellow thing that looks like a pin and a plus sign. Alternatively, go to the add menu and press the placemark item that comes down. (Hotkey: ctrl + shift + p).
Third, Edit your placemark. Move it to the right spot, preferably not on the object your looking at. Next, edit the description. Add some useful information, maybe some pictures, format it so it looks nice, etc. You do this with html tags, which aren't too bad, althought they look hard. (If you don't know any, google "Html Tags". The basic ones I use are:
and<a href="www.yourwebsitehere.com">Your Text Will Display Here</a>
Your location being a website, or a file on your cocmputer.<img src="location">
Fourth, save your placemark. Right click the placemark and press "Save place as". Save it as something easy, with no symbols or spaces, like "newyorkcitycab" or "plane_over_oz". Spaces often mess up files, but you can use "_"'s instead of spaces, usually.
Fifth, go to the google earth community website. If your don't already have a username and password, register on their website. Follow the process, and when you're done registering, you're ready to post.
Sixth, post that file. Find the appropriate category. If you want it posted in the layers, make sure you post in a (Moderated) labeled category. Say you found a plane in flight. Go to the "Transportation (Moderated)" category. Click "start a new topic". Write up all the information you want, people might want to see. Html comes in handy here again. Preview your post to make sure you like it. DON'T submit it yet. Go back, and add your file first. At this point, I don't have a GEC account, so you'll figure it out on your own. You'll have to upload your file somewhere. When you've done that, preview it one more time, then hit submit.
Seventh, wait. If you want your file to appear in the layer, you have to wait for some GEC admins to look it over, and for an update in their layer. These come every once in awhile. I haven't got a clue as to when, but probably one every few weeks.
Good luck, and happy posting.