How to contribute to an Open Source project

It depends a lot on the project.

Some projects have clear instruction on how to set up the local development environment and how to contribute. Some, usually the ones that are run by corporations, might even require you to sign some electronic document assigning the copyright of your work to them.

Others have no such information.

If you find the details then follow those.

Otherwise:

Try using the software and if you find any bugs check if the bug was already reported. If yes, you might want to expend on it. If not, this is your opportunity to open an issue.
Look through the open issues. Try to understand them and if you feel you can add to the conversation then do so.
If you’d like to add a new feature, look through the issues (both open and close issues) if the subject already came up and what was the conclusion. If it was not discussed yet then open an issue describing the feature and wait for the other developers to weight in.