The social media icons are available in the editor in the Company drop down menu below the placeholders, don't be afraid to scroll down a little bit.
If the icons of social networks we provide by default are not enough, you can easily add any icon/s you want.
We recommend getting icons from this site (used in the video) if you want to complement existing ones.
Otherwise, you can take a look at Iconfinder websites, which is a good source of nice social media icons of all styles.
If you want to connect this custom icon with a custom field:
Choose what placeholder your want to use for generating the link
Double-click the custom icon in your signature template
Click on the Link tab
Insert the name of the field (e.g. {customField2} )
Click OK
The icon now should have the link connected