Using skiner is really simple:
on first boot, you'll be asked to select your SD card from a list of all devices in /media
It is important that you select a valid SD card and not your ram or harddisk. Trust me, that gets messy

After that, if you change SD card, or want to move onto a second card, simply press space or A and reselect

Now you will be taken to the install screen, where you'll see all avalible skins listed, and a preview of the selected skin
Navigate this menu with the up and down arrows, and press enter or B to install that skin.
Once the skin is installed, the red selection bar will go green to tell you its installed

R activates uninstall mode. Here you can select a skin to uninstall with enter or B. L returns to the install screen.
This program will not stop you uninstalling your active skin, but dont worry, mini menu will reset to default if you do that.