You need access to your ftp server and an ftp program like
cuteftp or some such for easy upload. You upload the skin to
(generally) your forum root "images" folder or as per skin instructions. Then install the skin via your AdminCP ==> Style & Templates ==> Download / Upload Styles ==> Import Style XML File ==> Choose style.xml file ==> Import
At anyrate - I'll help you if you still need assistance. That being said, you should be extremely careful to whom you give server and AdminCP access to. You'd need to create temporary accounts with access to your forum folder (for ftp) and Styles & Templates via your AdminCP. Delete the temp accounts immediately after files have been uploaded and skin installed.
HTH,
J.