Installing a skin from the official repository
Many skins are available within the official repository which is already set up after installing XBMC/Kodi.
To enable a particular skin, go to System > Settings > Appearance > Skin. You should then see in the main area something like "Skin: Confluence" or whatever the currently selected skin is. Click on this option. In the next window, click on "Get more" and you will be presented with a list of available skins. Click on the skin you would like installed, and click "Install" in the next window.
After installation is complete, you will be asked if you would like to change to the new skin.
Installing a skin from zip
If the skin you're wanting isn't available in the official repository, then probably the best option, is to install from zip.
Download the zip file of the skin you would like, or go to the github etc. page and download as zip.
Once you have the zip file, put it onto a usb drive, or local drive so it can be accessed by your XBMC/Kodi device.
In XBMC/Kodi, go to System > Settings > Add-ons > Install from zip. Now browse for your skin's zip file and begin the install. Once the install is finished, you will be asked if you would like the change to the new skin.