I don't think I've ever known anyone to make mayo from scratch personally. I've seen it done on cooking shows though.
You just need one of this:
and the container that fits and comes with it (kind of like a tall glass)
+ one Egg (not too cold so let it warm up if you just took it from the fridge).
+ Oil part olive oil with some vegetable oil myself I like canola oil (just olive oil is just too strong).
+ Salt to taste.
That's it.
you can add some fresh garlic for more umph.
Chiplote or other hot peppers or other ingredients for flavor optional as well.
Start from the bottom and let it coagulate. while mixing it, forgot the percentage of oil right now but easy to find online.
Easy and cheap and way better than anything that you can find on the store. But it doesn't keep for a long time, one week tops refrigerated then throw it away, but we usually make enough for what we need so we don't have to worry about keeping it.