Just remember, 3 parts oil to 1 part vinegar.
It's a flexible ratio, so add more vinegar if you like things more tart or more oil for a richer dressing.
Just take an old conserve or jam jar, and eyeball your oil. Then add ⅓ the amount of vinegar. Add a pinch of salt and pepper. Here, you can also add a dash of mustard, some herbs like oregano, or lemon juice.
Close up the jar and shake the heck out of it.
Put your finger over the salad and pour a bit of the dressing over your finger and salad. Then taste the dressing on your finger. Adjust to your taste.