Open your DAW or audio editor. There are pros and cons about using each. By executing the action in an audio editor, you can save multiple versions of the file but won't be able to hear it in real time playing along with the song you've created. Executing in a DAW will allow you that luxury, but you won't be able to save that piece of vocal as its own file and therefore won't have a master to play with in case you want to apply a different effect to the same section later in the process.
Load the audio file. In a DAW, this will be in an audio track. If this is your first time attempting this, it would be best to make sure you back up the file in case anything happens that you didn't expect. You also could record a test file for experimentation purposes so nothing important is lost.
Highlight the audio file. If there's empty space at the beginning or end of the file, erase that. You don't want to stretch the space, which will only lengthen the file needlessly.
Find the "Time Stretch" command. The way to find the "Time Stretch" function varies with just about every DAW and audio editor on the market. It's important to familiarize yourself with the menus of your software and what each thing does. Time stretching is a common function that comes with even the most basic audio software.
Experiment with the different parameters. Again, depending on what program is being used, it will vary. What you want to look for is the length of the file and where that can be adjusted. This may be represented in seconds or milliseconds or the amount of samples for the file. You want to increase it by approximately three times to get the stutter effect. Listen to the file then save.