Lovely :D
I've never really used it myself, but I actually have it downloaded so I have the interface ^^
I can give a few quick shading tips :3
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When you are on the layers for the artwork, you can see it says "normal" and there's a dropdown there - there you can choose different layer settings :)
Now there are a lot of them but the ones I use the most are Multiply and Add - Multiply for shadows and Add for light!
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The way these layers work is that you have the multiply- or add-layer as a clipping mask on top of the layer with your colours, and then you choose a colour to shade with. I usually go for a light grey-blue for shadows but it depends on lighting :D
I'd suggest experimenting with it a bit and seeing what happens, overlay and soft light are also great layers for adding some ambient colouring to a piece!
Hope this helps a little bit, please ask if you wonder anything! I'm always happy to help <3