When I have downloaded a skin,what do I do next?
Where is the ccs? Or what should I do ::>.<:::

The CSS should be in a word document labeled CSS. Open that and in the ADMIN CP click MANAGE STYLE SHEETS. Then replace what's there with the new CSS.

Do the same with the HEADER AND BODY and the FOOTER.

^What he said. I know kman does that a lot with his skins. :)