This is just a simple oscilloscope. All you need is a mic. Have fun!
To use this as a tuner:
Click on the note you want to tune, to set the tuner.
When you play the note, you should see 5 peaks (by default).
If the peaks are slipping to the right, the note is flat, so tune it higher.
If they are slipping to the left, the note is sharp, so tune it lower.
Be careful that you are tuning the right string.
I am not responsible for any broken strings or guitars.
Guitar Tuning Tips:
To get the cleanest waveform, play the harmonic on the 12th fret. This is as close as you can get to a sine wave, the purest tone you can make. Since this note is an octave higher than the open string, you'll be looking for twice as many peaks.
Some sound cards have clock drift, which means they record slightly faster or slower than they should.
If you have an external reference like a tuning fork, you can set the note to the reference note, and adjust the calibration.
If you're using something other than your sound card to record, like a webcam mic, your audio playback might be a reliable source. For example, I discovered that my Logitech Quickcam Mic needs a calibration of 0.9978.
If you are using separate devices for playback and recording, you can calibrate by playing a tone through your speakers, and adjusting to that.
Width in cycles:
Click the note you want to tune to.
Make sure you choose the right one, or you could break a string, or worse. All the tuning are low to high.