Much better than the one you buy and very simple
1 egg at room temperature
1 tablespoon Dijon mustard
1 tablespoon white wine vinegar
1/4 teaspoon salt
240 ml neutral flavored oil
Add egg to the a ceramic bowl. Add the mustard, vinegar, and salt. Beat with a fork for 1 minute.
Slowly add the oil in tiny drops until about a quarter of the oil has been added meanwhile mixing energetically with a fork by way of wrist (this is critical for proper emulsification).
When you notice that the mixture is beginning to thicken and emulsify, you can increase to a very thin stream instead of drops of oil.
When all of the oil has been added, taste mayonnaise for seasoning then add salt or extra vinegar to taste.