How do I add a solder mask?

The solder mask is automatically generated for Gerber production, but you can add it is a layer.

Launch the Design technology Shift + T and click 'add' and fill out the details as shown in attached.

 

Mover layers in the stack for the correct order they physically appear in. (Note 'documentation' layer is wrong in attached image it should be right at the top).

In the interaction bar you will see your new layer as shown.