Make files writable

Hallo,

Iam just starting and am trying to edit the appearance. firts I need to make a file writable. I have edited the css 7 index pages as described in the Codex, but o luck. It also says:

For core WordPress files, all should be writable only by your user account. However, if you utilize mod_rewrite Permalinks or other .htaccess features you should make sure that WordPress can also write to your /.htaccess file.

If you want to use the built-in theme editor, the theme files you want to edit may need to be group writable. Try using it before modifying file permissions, it should work.

This I dont understand.

can simebody please explain step by step please?