how to reference a plane at a 3D angle to a model

I've hit a wall with a model! I'm trying to find out how to create a plane to draw on at an angle to an already existing plane? For example, trying to send a drawing backwards and down at the same time?.. it might be something completely idiot proof and trivial but I just can't think?... can someone help me out or point me in the right direction to something that can?

Cheers!

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