The idea here is to mix a quantity of some oil with a lesser quantity of some aqueous acidic solution. These two liquids don't mix well so some kind of emulsifier is usually included. The commonest mixture is probably olive oil, wine or cider vinegar and mustard powder. This is what's usually called French dressing. You can vary the oils, use lemon juice or soy sauce instead of vinegar and use other ground spices as emulsifiers. You can also add chopped herbs, crushed garlic and other flavourings. The list is endless.
Here, I'm going to give you the quantities and technique for French dressing, then I'll suggest a few combinations of alternative ingredients to get your imagination going. The basic principles are the same whatever you use.
These quantities are for one large salad. Scale them up if you want to make enough to store.
Ingredients
3 tablespoons of olive oil
1 tablespoon of wine or cider vinegar
A large pinch of mustard powder
I don't include salt as it can react with the acid to make a very sour dressing. I prefer to mix a little salt into to the salad just before I add the dressing.
Put everything in a small jar or similar container, put a lid on it and give it a good shake. Pour the contents over your salad. If you leave the dressing lying around it will separate into layers, but that's okay. Just give it another shake and it will be as good as new.
Here are some ideas for variations.
Oriental style
sesame or peanut oil / orange juice and soy sauce / ground ginger and coriander
Italian style
olive oil / wine vinegar / ground black pepper plus crushed garlic and chopped
fresh basil
Arabic/Indian style
sunflower oil / lemon juice / ground cumin and chopped fresh corriander