Generally, you can use any audio programs that allow you to import files, like Audacity and GarageBand. You'll have each of these covers saved as mp3, wav, or any other type of audio file (depending on what the program/computer allows). If you're using Audacity (which is free to download. If you want, I could give you the link), you'd start by opening the program and selecting "Import". You'll then find and select the cover's file and insert it into one of the tracks. After that, you repeat it with all the covers you've saved. On Audacity, you can adjust the timing on the tracks (like moving the start to be a few seconds later). Once you've got it all ready, you'll go to "File" then "Export". Save your file. At this point, you'll just need to edit in the video(s), which can pretty much be done on iMovie (and a lot of free movie making apps, really). I hope this helped you out! If you have any more questions or want me to elaborate, don't hesitate to let me know!

Thank you very much for the detailed response! I actually have just downloaded Audacity recently, though I don't really know how to use it well. I actually have a difficult time doing certain tasks without having a visual guide, so if you could I'd be very grateful if you could explain in further detail with screenshots if possible

So first, you'll want to open up the app (I'm on Windows, so it may look different depending what kind of PC/Mac you have). Then, in the top left corner, you'll see the toolbar (if that's what you call it), and "File" should be the first option there. Click on that, and a dropdown should reveal a few more options. Go down the list and hover over "Import" (for me, it's the eighth one down). This should give you the choices of Audio, Labels, MIDI, or Raw Data. You'll want to click on "Audio". Once you're there, you can select your first audio file. After you do so, it should appear as a track, with the file name in the upper left hand corner. From there on, you'll be repeating those steps until you have all the covers you want for your chorus. Now it's time to align them. Next to the play button, there should be 6 different tools. The default is the first one, which allows you to skip around/listen to different parts of the track. Go to the one with arrows on each end and select it. This will allow you to move the tracks around, adjusting the timing and making for a clearer sound. Though it won't be necessary, there are some effects (under the "Effects") tab that you can use. To do so, you'll need to pause the track (the grey square next to the play button).

I can only add one image per comment, so below I'll add a few images and explain what they are. If you need anything else, please let me know!

This is what the screen should look like when you first open up the program.

Here's the toolbar. "File" is the first one (at least it is on my computer). That'll be right up at the top of your screen.

I couldn't get a shot of the "Import" button, but it shouldn't be too hard to find once you select "File". After you've imported your first sound file, it should look something like this.

Then, you'll add in the rest of your files the same way (I only have two in this image). As you can see, the sound alignment on them is a bit off.

The move tool to adjust them can be found in this area.

It's this one.

If you want to adjust the sound on each track, it's the top slider here (with the + and -). The second slider will dictate which side of your headphones/earbuds/speakers the sound will play from: left or right. These will be on the side of each track. The main sound and panning can be adjusted on the sliders next to the tools (you can see it 2 images up).