Installing from source, if you are not enrolled in Apple's Developer program (and thus have no Safari Extension Certificate), will only work for the lifetime of Safari. You'll need to do this every time you start up Safari, it is therefor recommended to install the binary below.
- Open Extension Builder through
Develop
>Show Extension Builder
- Click on
+
>Add Extension
- Locate the source code folder called
HerpDerp.safariextension
and open it - Click
Install
Download the binary from here (Thanks to @gggritso for building it). Simply double click it to install it into Safari.