In Studio, from the toolbar choose 'Background', then 'Load Tracing Image'. In Companion, choose 'Pic' then 'Load Tracing Image'.
It's pretty straight forward, it just loads up a picture and you draw over it. When you save the picture resource, the trace image isn't saved with it. If you need to do this in multiple sessions, you can reload your tracing image after opening up the pic resource that you are working with.