Wikipedia

For anyone interested in the actual mechanics on how to do this:

There are two things you must do in order to nullify:

1. You must get on a jury.

If you are called for jury duty, we recommend visiting our online guide Called for Jury Duty? to learn about jury selection and how to maximize your chances of being selected as a juror. It covers topics such as appearance, behavior, and how to answer questions during the jury selection process. Doing a little bit of preparation in advance will increase your odds of being able to save someone from being punished unjustly.

2. Vote ‘Not Guilty’.

If you have a conscientious belief that acquitting the defendant is a just verdict, even if you believe he or she has technically violated the law, there are only two words you need to know: Not Guilty.

In recent years we have seen some people suggest that you must identify your intention to nullify in order to do so. PLEASE DON'T! That is one of the worst things you could do. If a judge determines that you are considering not enforcing the law (for example, if one of your fellow jurors complains about you to the judge), then even as late as deliberations you can and most likely will be removed from the jury. This most likely will leave the defendant with no conscientious juror ready to contravene the judge's instructions to convict against their best judgment of what would be a just verdict.

We recommend not openly discussing jury nullification during deliberations unless it is clear that several other jurors are also openly considering it. If there are too many such jurors for all of them to be removed and replaced by alternate jurors, then the most likely outcomes are either a mistrial or an acquittal. If it is just one or two jurors thinking along these lines and they can easily be replaced by alternates, the most likely outcome is that they will be replaced and the defendant will be convicted.

While you can be removed as a juror even as late as deliberations for indicating your intention to nullify, you cannot legally be removed for expressing doubt that the defendant is guilty. Neither are you required to explain your vote. You can participate in deliberations by expressing doubts about the defendant being guilty if you have them, asking questions, listening actively to your fellow jurors, and so on. If you feel the need to explain your vote, you can say something general such as that in your heart you cannot convict the defendant.

https://fija.org/library-and-resources/library/jury-nullification-faq/how-do-i-conscientiously-acquit.html

A much more in depth guide:

https://beyondcourts.org/sites/default/files/2022-07/Jury-Nullification-Toolkit-English_0.pdf