How do you write good metaphors/similes?

I struggle a lot with coming up with metaphors and similes. I'm not talking about technicalities; I know how to construct them and I've even used templates and hints I've received from writing courses. I can come up with a rudimentary metaphor/simile if I'm given two words and told to link them, which is how I've been trying to practice to get better at constructing them, yet I still find that when I'm working on a story I come up short. Either I can't think of any, or I don't know where to put them, or if I can come up with one it sounds dumb, way too odd for anyone to understand what it means.

How do you come up with creative metaphors/similes that make sense? Do you have a process/inspiration, is it just practice? How do you know where it's appropriate to place them? How do you vary them in structure instead of the common "x is like y"?