1. Download the Rainmeter Application to your computer from HERE
2. Install the application.
3. Once you installed the Rainmeter application, by default illustro skin will be applied.
4. In Manage Rainmeter window, you can see the skins which you installed. Expand the skin folder and then expand Gadget folder, then select the .ini file and click load to appear on your Desktop or unload to disappear the gadget from desktop.
5. Like this if you have multiple skins installed, then you can load one gadget from one skin another from another skin.
6. To download the skins, go to Deviantart or Customize.org.

Note that, the skins will have the extensions rmskin which are directly executable and some have zipped. If you download the Zipped skins, extract them and place the folder in Rainmeter skins folder (generally, C:\Users\YOUR_USER_NAME\Documents\Rainmeter\Skins). In Manage window, click Refresh to appear your skin folder.

All the skins you installed are found in Rainmeter skins folder, if you delete particular folder the skin will be removed. The folders present in Rainmeter skins folder only are appear in the Manage Rainmeter window.

Check out my rainmeter skins:-