Honestly, if I'm lost, I often go to Chinese to see what they do for abstract elements. I was completely lost on length in High Valyrian, for example, without being able to compare one thing to another—and especially with the whole "banana for scale" problem. The idea is so basic, though, I figured it would be a basic term in Chinese that would have its own glyph, and I was right! Turns out in Chinese the origin derives from hair. And that, of course, is brilliant, because human hair is something you can depict and has an absolute reference (i.e. at a certain point, all humans will agree that a certain length of hair is long).
At that point, though, I did my own drawing for a person with long hair, and used that as the source of the glyph in Valyrian. There's nothing wrong with using the same etymological source as long as you do your own work from there. It's a good thing to do when stuck.
As for other things, though, you go through the same process. It's easier to make images of concrete elements than abstract. When it comes to abstract concepts, think about what concrete elements in your world are most closely associated with those abstract concepts, then do glyphs based on those concrete things. You should also feel free to take combinations of elements and combine them into a single glyph (for example, maybe your glyph for "warm" is a combination of hot + cool, or hot + small). Always start with the concrete elements of your conlang's world. Also, the associations don't have to make perfect sense, so long as they make sense to your speakers. After all, the same thing happens with word etymologies.
Anyway, hope that helps! I also recommend following others who create logographies like my friend @yuk-tepat. He's really good, and you'll learn a lot by reading his stuff.
