Don’t buy that vinegar-y crap in a jar. Get the hang of this and you will feel that you Rule The Cooking World.
Get a bowl and put an egg yolk in it. Put a teaspoon (yes that much) of EITHER dijon mustard, OR posh vinegar. Either of these helps it become thick and not curdle. I also grind in salt and pepper at this stage.
Get a small whisk and with your right hand (if you are right handed), whisk like mad, and with your left hand, drizzle VERY slowly some drops of ordinary oil like light olive or rapeseed or sunflower. Purely using extra virgin olive oil will result in a overly-bitter mayo. The ONLY important thing to remember is not to glug it in, but to drizzle like sand in a tiny egg timer and to take lots of breaks to incorporate all the oil as you go, beating beating beating… If you’ve added it slowly and gradually enough, after a minute it starts to get a bit thick, like this:
Keep beating and keep slowly stop-start-drizzling the oil, and another minute later it looks like this:
Now you can probably start to add the oil a bit faster, as you’re past the stage where the egg will split. So it’s not like putting oil in a car engine, not glugging it, but not wrist-achingly slowly like before. And then after another 30 seconds it will get really thick like this:
Now it’s often almost too thick to beat any more, so I usually add the lemon juice at this stage, to loosen it up. Add the juice from a thick slice of lemon and beat it in a bit. It now starts to look like the thing you get in the jar, except more yellow because it’s real egg. You can go back to adding oil now, if you need to make a larger amount of mayonnaise. The egg will carry on taking the oil, as Heston Blumenthal once proved, almost to infinity. But oil is expensive and you don’t want a massive bowl of mayonnaise necessarily, so the beauty of this little homemaking skill is that you make as much as you need.
Add salt/pepper/more lemon/more mustard to taste.
Feel smug. Your five minutes have been used wisely.