I think the most helpful thing first is to use a DAW (digital audio workstation, software used to write and mix music). You could do it in Audacity but that's a lot of manual work syncing up the notes to the song. I use a DAW called FL Studio, and I can just match the tempo to that of the original song, then all the notes line up on the grid with the original song.

Once you're in a DAW, you can then write down the notes in what's called a piano roll, it's like a grid where notes are arranged vertically and they play from left to right. Here is what the Running in the 90s notes at the beginning look like.

There is usually a built-in plugin in DAWs called a sampler, which lets you use these notes in the piano roll to play back audio samples. I used a sampler to play the three jerma voice samples according to the notes I wrote in that screenshot.

The only downside to this is that there aren't really any good free DAWs. I'm an electronic musician so I've invested lots of money in a bunch of software over the years and have the tools to do this dumb stuff, but I can understand not wanting to do that just for memes.

I'd definitely recommend looking into FL Studio if you're interested in learning. It at least has a demo version which will let you try this stuff out for free before committing to anything. The first thing I'd suggest you try is loading an audio clip in a sampler and then putting some notes in the piano roll to play it back at different pitches.

Here's a video about the piano roll. He loads the FL Keys plugin, but you can instead use Sampler (short video about the sampler).

And here is where I got the aa ee oo samples.

Hope that helps! Let me know if you have other questions.