Fussypants

Shading as in where the shadows fall or shading as in how shadows transition from light to dark?

yes.

right now just a sketchpad and pencils of varying hardness. I'm thinking it may be easier to bite the bullet & go digital.

I don't work digitally so I don't know about shading there, but I do know traditionally, pencils are wonderful for it

It's all about practice and references. Looking at how the light falls goes a long way towards being able to draw it :)

I've been doing eg, 'upside down photo' exercises, but shading exercises are super 'now draw the rest of the owl' for me.

Upside down photo exercises? I've never heard of those before, I don't think

turn a portrait upside down, 'draw what u see, not what u know.' Easy to see shapes, rotate back to see where u messed up.