Whisk egg yolk, mustard, and Acids (citrus Juice + vinegar) and salt until completely blended.
Add just one drop of oil, and whisk until fully blended again. Repeat 4 more times. Each time ensure that the mixture is fully blended before adding the next drop.
Now add half a teaspoon of oil, whisk until blended. Repeat 4 more times. Each time ensure that the mixture is fully blended before adding more.
Now the mixture has enough oil emulsified that you can slowly add a stream of oil while whisking and the existing emulsion should not split.