Attach the guitar tuner to the guitar. If you use a tuner that clips onto the guitar, put it next to the tuning keys. Some tuners only require that it be nearby the guitar to get accurate readings.
Start tuning the thinnest string first. Tighten the tuning key while continuously plucking the string until the tuner reads B. Since your ears can sometimes pick up imperfections that the guitar tuner may not notice, you should use your ears to do the fine tuning. The pitch should be constant and not bounce around.
Move on to the second thinnest string and tune it to E. Strum the two strings together and listen carefully to make sure they are tuned properly. If the two strings are not in harmony then double-check that they are correctly tuned.
Repeat the steps for the remaining strings, working from thinnest to thickest. The entire order of the strings should be B-E-A-D-G-B-E when finished. If it was tuned properly, all of the strings should be in harmony when strummed together.