All you have to do is download and install Rainmeter and the skins you want, et voila!
Also, a lot of the skins themselves are customizable, though the ease of doing so varies from skin to skin.
Some, like Elegance 2 (the clock I used in Beat) have easy to use menus to change features (in this case, color).
Others, however, require going into the .ini files (the file that holds all of the programming) and mess with the coding to get the result desired. This often involves a lot of trial and error, so make sure you save the original code in case you need to go back to it, otherwise, you'd have to reinstall the skin. I did this with Elementary's chrono to add the time and fix the wording and such.
I know this is really long, but I hope it helps!
If you need any more help or tips, feel free to send me a message!