The accepted wisdom on making this is three parts oil to one part vinegar (or citrus juice). I tend to amp up the oil proportion a bit because, while I love sharp tastes, I prefer that the flavour of the oil take precedence.
This is an unnatural marriage. Oil and vinegar do not mix but can be combined in a temporary "emulsion"by whisking or shaking together. It will separate out later but can be reshaken.
1. Take three parts oil. (Your choice. Remember olive oils all have their own particular flavour.)
2. Add 1 part of vinegar.
3. Add salt and pepper.
4. Add 1 tsp of grain mustard for every cup of liquid.
5. Add 1 tsp of sugar for every cup of liquid.
Shake well in a screw top jar.
Shake well again before using.