If your yogurt is real watery, place it in a coffee filter lined strainer for 30 min to get rid of the excess liquid. If it is a newly opened container of yogurt, you most likely will be able to use it straight up.
Place the plain non fat yogurt, light Miracle Whip, dill pickle relish and chopped onion in a small bowl. Mix together.
Add salt and pepper to taste and mix in.
If you choose to add lemon juice, add it now and them mix it in.
Cover the bowl with plastic wrap and refrigerate for 30-60 min so the flavors can meld. Serve when ready.
The tartar sauce is best when used on the day it is made. Leftovers can be stored but it will get watery as the yogurt continues to give up whey.
Notes
Plain non fat yogurt was used to reduce calories and fat. If you want to make a quantity of tartar sauce to keep on hand for 3-5 days, then I would recommend replacing the yogurt and light Miracle Whip with an equivalent amount of light mayonnaise and adjust the WW points accordingly.