Connect an XLR cable to your microphone. Connect the other end to the "XLR Input" jack on an audio interface.
Connect the audio interface to a computer. The connection method varies according to what make and model you use, but typically calls for either a USB or Firewire cable. Connect the cable to the relevant port on the computer.
Connect a 1/4-inch jack cable to each of the "Speaker Out" jacks on the rear of the audio interface. Connect the other end of the cables to either powered speakers or, if you're not using powered speakers, to the speaker amplifier.
Open your preferred digital audio workstation --- for example, Mixcraft or Logic. Double-click the desktop icon. Typically, the last opened session automatically loads. If it does, click "File" and select "Open New." When prompted, close the other session.
Click "File" and select "New Audio Track." Choosing audio, as opposed to MIDI, assigns the audio interface as the primary input device. All effects subsequently applied to the audio channel influence the sound of the microphone.
Name the audio track. Double-click where it says "Audio 1" and type "Autotune vox." Naming the channel makes saving it as a preset quicker, should you decide to do so.
Click "Effects" and select "Autotune." Auto-tune is a trademark owned by Antares, the company that developed Auto-tune. It is distinct from "Autotune," which is frequently used to describe the general sound of a pitch-correction tool. If you don't have the official version of Auto-tune on your DAW, open the stock pitch-correction tool. For example, in Logic this is called "Pitch Correct" and in Mixcraft it is called "GSnap." This applies the pitch-correction tool to the audio channel in real time. Whatever you sing into the microphone will come out with the effects on.
Click the "Key" button on the top of the interface and enter the key that your song is in --- for example, "C Major."
Adjust the pitch-correction parameters to your taste. For the classic Autotune sound, set the "Retune Speed" to zero. This makes for an unnaturally fast pitch-correction, where the note sweeps rapidly from the original pitch to the new pitch.