Follow the instructions in the Oxygen Builder website : User Design Library
Then do the following:
1 - Open Oxygen builder
2 - Click on the button Add then select Library
3 - Choose your Design Set
4 - Choose one or several elements from the Sections & Elements
5 - Play with them, study them, check the Code Block to find the CSS, experiment, have fun
6 - Customize your own element and when it's done, copy the custom CSS from the Code Block and paste it to your Stylesheet.