when you download the new skin, you'll see a temp folder come up containing a texture folder, a read me test, and usually a bitmap or gif image to show you what you're getting. it will look a little like this:
highlight the texture folder, then go to the "edit" pull down menu, and choose "copy to folder" this will bring up a little explorer window. navigate through your drive to microsoft games/flight simulatorX/sim objects/airplanes/ then choose the plane where you want to add the skin. click "copy". next, open the read me file with notepad,
AND ACTUALLY READ IT. many people don't, and then wonder later why things don't work properly. there may be things in there you need/want to know. usually there is. anyhow once you open it, scroll down until you find the part i highlighted below:
highlight the text, and then hit ctrl + c to copy it to your clipboard.
hit the windows button + E to bring up windows explorer. in the left panel, navigate to your drive, then to
microsoft games/flight simulatorX/sim objects/airplanes/ the airplane you added the skin to
the contents of that planes main folder will be displayed in the right panel. find the aircraft .cfg file, and open it with note pad. you see paragraphs nearly identical to the one you copied to your clipboard. there will be one for each skin that you already have. the first one will be numbered as ZERO not as ONE. scroll down to the last paragraph for skins, and take note of the number. skip one line under the last part of that paragraph and hit ctrl + V to paste the text you copied earlier. whatever the number of the last skin was, add one to that number, and replace the X with that number, at the top of the text you just pasted in. save the changes, close all your open windows, and start fsx as you normally would. when fsx opens, look in the aircraft select window, and you should see a new thumbnail pic representing the new skin.
on the off chance that you downloaded an old skin that has no thumbnail, you'll see a little square with a question mark. select it, that's your new skin. i hope this helps make it a little more clear. just remember every single person on this board was a noob once and had to learn the exact same way you just did.
don't worry, you'll be a pro in not time at all, it's not so hard.
if that doesn't work for you, contact me and i can walk you thru it with a chat window.