Yes, just put it into a file "YourSkinName.ascx", and use FTP to upload it to your server (or windows explorer to copy it to your local DNN). You dont need to zip it. Note that this very simple skin will still have a control panel when logged in as admin, since DNN auto adds it if it is not in the skin.
You can either put it in:
/Portals/_default/Skins/YourSkinNameHere/YourSkinName.ascx, or
/Portals/0/Skins/YourSkinNameHere/YourSkinName.ascx (change "0" to your actual portal id)
If you put it in _default, then to use it, on the site settings page choose "Skins - Host", otherwise choose "Skins - Site", and it will be in the drop down list for you to select.
If the skin is too boring you can start to improve it with some CSS, in which case you need to create a seperate skin.css file and upload it to the same directory as above. Then add the class name to your skin...
<div class="main_content" id="ContentPane" runat="server"/>
The skin.css file would then contain:
.main_content { font-family: comic; font-size:12pt; } (or whatever styling you care for)
JK.