Before start this job be sure to turn off the water main and drain the pressure from the lines.
Well, you have replaced the washers and find the faucet is still dripping. Then the issue is most likely that the seat that the washer presses against to stop the water, has gone bad.
Replacing the seat can be a little tricky because you are working inside the wall. With the correct tools and a little patience you can accomplish the job.
You will need the same basic tools as explained on page 1. Plus the addition of a seat wrench. You can pick up a seat wrench at your local plumbing store where you purchase the seats.
For guidance on getting this done, check out the video below.