General modding

I know this is a bit awkard asking this but as i have no hope of doing it myself i really do need alot of help. All i want to know is how to mod on any game, I do realise it depends on the game and this is a MTW site/forum so this will only be an example. Say if I downloaded some new skins for Star Wars Empire At War, how would I go about putting them in. And please, if you are awesome to help me computers and I arent really best friends so you will have to give pretty simple yet detailed advice. I do thank alot all those who do end up help and I do apologise for not being quite as skilled as you.