Bottled ranch dressing doesn't have hardly any carbohydrates, but will almost always be high fat and/or high sodium. There are some ways to fix this, but first, as for any food item, it's important to figure out what the key herbs and flavorings are, and what the main base is. Separate those in your mind, and try to figure out how to make each component better.
Ranch typically has a cream base. Here, you can substitute cream for fat free buttermilk, fat free plain greek yogurt mixed with a little water, or a combination of fat free sour cream, light mayo, and some nonfat milk.
The basic flavorings for a homemade ranch are dill, parsley, chives, onion powder, garlic powder, salt, and pepper. Whisk these into your base to combine.
Instead of salt, add parmesan or pecorino romano cheese for flavor with less sodium.
Pour everything into a jar.
Add the lemon at the last moment, shake to combine, and refrigerate.
Here's the ingredients list to proportions:
1/2 cup light mayonnaise
1/2 cup fat free sour cream
1/2 cup fat free buttermilk
3/4 - 1 teaspoon dried dill
1/2 teaspoon dried parsley
1/2 teaspoon dried chives
1/4 teaspoon onion powder
1/2 teaspoon garlic powder
2 tbsp finely grated parmesan, or for a stronger flavor 2 tbsp grated pecorino romano