How does it look if I use the wrong reading?

[Leebo]

It depends. Sometimes if I accidentally switch the order of two onyomi my girlfriend can still guess what I meant to say. But if you toss in just one completely different reading like that, it probably will be tougher to understand.

@Facade

I tend to think about words in terms of parts that make up the word, so I use the wrong reading sometimes, even though I know both readings of the kanji that make up the word

For Japanese people, it’s the complete opposite. They first learn the words, absent any of the written context, and then they learn how to write them later. So they’re not doing the same kind of stitching together of readings.