How to wear a belt to look slimmer ...

Belts are like statement necklaces, they should only be worn to draw attention to an area you would like people to look at.  So, if you are a full hourglass with enviable curves, like Marilyn Monroe, you definitely need a wardrobe full of belts to show off your waist!

The key to wearing the right belt is to ensure it's in proportion to your overall build, a very thin belt worn over clothes can make the wearer appear larger than they are:

Whereas a thicker belt can be much more flattering, especially if worn with an open cardigan / jacket:

Not everyone suits a belt; if you don't have a defined waist you're only bringing attention to the fact you don't have a waist.  Concentrate on your best features and use colours / patterns / accessories to bring attention to those.