“A good rule of thumb is to stand with your feet shoulder width apart.”

You're teaching your niece to play golf. Her stance isn't good, so you're giving her some pointers. You say:

A good rule of thumb is to stand with your feet shoulder width apart.

a rule of thumb

A "rule of thumb" is an easy-to-remember rule that you can use to make a decision. 

Some useful rules of thumb include:

• Don't date someone that you work with.
• Try to get 8 hours of sleep each night.
• Put at least 10% of your pay into savings.

"Rules of thumb" aren't always true, but they're true most of the time, so you can follow them unless you have some better information to use instead.

The phrase "rule of thumb" is most often used in sentences in two ways:

I don't date people I work with, as a rule of thumb.

A good rule of thumb is to put 10% of your pay into savings.

stand with your feet shoulder width apart

Standing "shoulder width apart" means that you're standing with a little space between your feet. When you stand this way, each foot is straight below the outside of your shoulders.

This phrase is used in sports, exercise, and dance when you're giving instructions.