So shading has always been something that's a bit of an enigma to me- I can understand shadows well enough and I feel like I'm on the road to eventually understanding when and where to put them, but I really just don't get how highlighting (or whatever the opposite word you'd use for shading) works.

This may just be my lack of understanding forms speaking, but I know you put shadows wherever the ambient light doesn't hit, or in the case of bouncelight, doesn't hit as strongly. It can also be used to distinguish which side of an object isn't facing the viewer/light source.

I'm going to focus on cel-shading styles for now as I similarly don't understand color blending/digital painting, and my personal ideal style would be something like a mix between the mario strikers and sonic free riders art.

Needlessly long pre-amble aside, my question is in cases of "character standing in a void" or "assume the light source is the viewer" drawings, where would one put the highlights, and what methods are there for cel-shading highlights?

Using one of my recent drawings as an example, I currently do hatching in a multiply layer wherever I think shadows should be.

The thing is, in drawings like this, I wouldn't know where to put the highlights, even though I understand the forms, mostly.

Any recommended tutorials or reading I can do on this sort of thing would be appreciated, thank you for your time.

The simple answer is that highlights go on the opposite end to shadows, they're the parts that are getting hit with or reflecting the most light from your source. In the case of your drawing, the light source seems to be above and between the two characters (cool character designs btw!), so highlights would go on the upper and inner edges.

You mainly want to pick out important edges, especially ones that need a bit more contrast, and anything that sticks out, like noses or buttons or creases in fabric. Keep in mind that some materials respond differently to lighting, things like metals and skin are more reflective than rough fabrics and tend to have sharper/more intense highlights as a result.

I personally learned to do highlights from miniature painting, that skill transfers over surprisingly well, although I haven't incorporated them into much of my published art so far so I don't have a ton of expertise to share beyond that lol.

Stuff like putting the highlights opposite of shadows (that sounds obvious now that it's been pointed out to me, thanks) and focusing on edges that need more contrast/are important are the exact kind of things I was looking for, thanks. Are there any other tricks or methodology to choosing where highlights go, or are those the important ones?

That's as far as my knowledge goes, I am sure there's more tricks but I haven't figured them out yet lol.

I will add though, like with most things, a reference is good to have. You can grab a flashlight and shine it at stuff from different angles to see where the brightest spots are, usually they'll be along corners or raised elements but it varies a lot depending on the material and the shape of the object.