If you're not registered or you want to make sure all of your information is updated so you actually get a ballot, go to this link right here.
If you don't have a state ID, you'll have to register by mail or in person.
(The deadline for that is October 31.) If you don't currently have a residential address, you can use the address of a shelter, park, RV, intersection, or "other identifiable location." You can receive your ballot at a shelter, friend's house, or by general delivery at the post office.
If you want help keeping your address confidential, click here.
Even if you're planning to waste everyone's time and cast your ballot for Jill Stein (stop), there are a ton of non-presidential reasons you need be registered if you live in Washington.