When playtesting games at the Engagement Lab, most of the time bugs and other issues will be logged on Github. On the project page, go to the "issues" tab, and click the green "new issues" button.
When making a new issue, there are two main parts: the title, and the body of the issue. The title should be as clear as possible, detailing the bug without getting into unnecessary details that could be left to the body. The body is where all of the meat of the bug or issue lives and should include steps taken to reproduce a bug, screenshots taken of the bug/typo/style issue, and, if possible, an explanation of what seems to be causing the bug or issue.
When writing up steps taken to reproduce a bug, think of it as if you are writing a scientific procedure for a lab experiment. You want the person looking at your issue to be able to follow the steps exactly as they are written and easily reproduce your bug. Screenshots should accompany steps, but not be left without explanation so that it is clear what they are showing. Finally, if it seems clear to you when writing out steps and reproducing the bug what is the underlying problem, it is important to state that - even if you are wrong, it will help developers understand what the problem looks like, making it easier for them to debug and fix the issue.
Below are some examples of clear step-writing and explanation of what be the underlying problem in issues written in the past:
On the last issue example, on the right, there is a section for "Labels" that is also complete with different tags, in this case detailing the devices on which the bug occurs, and that the issue is in fact a bug, rather than a UX/UI issue or something similar.