Here's how I prepare it (but there are as many ways to prepare Vinaigrette as there are cooks out there):
- 1 tablespoon of Dijon Mustard (try "Maille" or "Amora" brands)
- add salt and freshly ground pepper
- add 1 tablespoon of red wine vinegar (not balsamic)
- stir well then add 1 tablespoon of canola oil, stir until having a homogeneous emulsion
- add 2 or 3 tablespoons of olive oil, one at a time, and stir well. If you add the oil too fast it won't incoporate to the dressing so make sure to stir well but not necessarily fast. Small movements work best, I think, but you should try to find your own technique
- try the dressing and add whatever is necessary (oil, salt, vinegar...) to make it perfect for your taste. Note that this dressing should be a little thick (not too oily).
There are many variants to the Vinaigrette dressing. You can mix different oils or use a flavored vinegar depending on what's in your salad (for example: add whole walnuts to mixed greens and replace one table spoon of olive oil by walnut oil). You could also add a finely sliced green onion or chopped chives. As long as you stick to Dijon mustard + any kind of vinegar + any kind of oil + optionally herbs, you can consider your dressing to be a vinaigrette...
Tip: if you leave the dressing in the fridge the mustard and vinegar will separate from the oil. You can get the emulsion back by stirring slowly in the center of the bowl, making small circles until part of the dressing starts to emulsify, then widen the circles to incorporate a little bit more of the oil until the whole batch of dressing is smooth again.