A simple vinaigrette is usually 3 to 4 parts oil to 1 part acid, like vinegar or lemon juice. A practical starting point is 3 tablespoons of oil to 1 tablespoon of vinegar, plus salt, pepper, and a little Dijon mustard if you want the dressing to stay mixed longer.
Whisking or shaking turns the oil and vinegar into a temporary emulsion, which is why vinaigrette looks smooth right after you make it and then separates if it sits. That is normal. Taste it before you pour it on the salad: more oil makes it softer, more vinegar makes it brighter, and mustard helps the dressing hold together instead of breaking immediately.
If you want a sturdier version, add a little minced shallot, garlic, or honey, but keep the same oil-to-acid ratio so the dressing still behaves like vinaigrette and not thin soup. Shake again right before serving, because the separator is gravity, not a mistake in the recipe.
A reliable dressing shaker is a small investment that makes this easier: dressing shaker.
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