Series 60
To install a new skin, send the two skin files (a PNG file and a TXT file) to the phone. Then, while they are still in your Inbox, run Bemused. Select "Install skin" from the Settings view. The name of the skin you want to install should appear in the list of installable skins. Once a skin has been installed, it can be deleted from your Inbox.
UIQ
To install a new skin, you can either send the two skin files (a PNG file and a TXT file) to the phone's Inbox over Bluetooth, or copy them using the PC connectivity software. If you choose the second option, you need to copy the PNG file to the folder "Images\Bemused skins" on either the internal memory or the Memory Stick. The TXT file needs to be copied to the folder "Documents\Bemused skins". Then, in Bemused, select "Install skin" from the Edit menu. The name of the skin you want to install should appear in the list of installable skins.
Once a skin has been installed, it can be deleted from your Inbox or "Bemused skins" folder.
New skins can be downloaded here.