I contribute to an open source project by doing extensive testing. I've been doing this off and on for more than two decades. I don't fix the bugs I find; the people who do are much better at that than I am. But I like to think I've gotten good at finding bugs, and try to find new ones as soon as possible so they can be fixed before formal releases.
I include (1) minimized input that can reproduce the bug, (2) a copy of the error and stack trace, or of an incorrect output, and (3) identification of the specific version of the software (a git id) and the architecture it was compiled for (typically x86-64).
