When donning formal dress, such as a dinner jacket, waistcoats are a common accessory. When dressing for a formal occasion, it is essential that all an outfit's elements -- from the trousers to the shirt to the jacket to the waistcoat -- are worn correctly. Part of wearing your dress clothing correctly involves ensuring a dress waistcoat's belt or strap has been properly adjusted. An improperly adjusted vest can make you look sloppy, or larger than you are.
Put on the trousers and dress shirt you will wear with the waistcoat. Ensure the trousers are already correctly adjusted. To ensure the vest belt is correctly adjusted and is looking as good as possible, it's important the clothes the waistcoat will be worn with are fitting properly.
Lay the waistcoat down on the flat surface. Ensure the waistcoat is flat on the flat surface without wrinkles, and that the back of the waistcoat with the belt is facing up.
Disconnect both sides of the waistcoat's belt. The left side comprises a strip of fabric and the buckle, while the right side is only a strip of fabric. The buckle is two pieces: a square or rectangle piece of metal, along with and a single piece of metal located in the centre of the square or rectangle piece of metal.
Pick up the left side of the belt with the buckle with your left hand. Pick up the right side of the belt with your right hand.
Pull the belt's right side under the buckle's right side. Pull the belt's right side over the middle of the buckle, the metal bar separate from the buckle's square or rectangle metal piece. Pull the belt's right side under the buckle's right side until there is about 1.2 to 1.8 cm (1/2 inch to 3/4 inch) of the belt's right side pulled beyond the buckle.
Put on the waistcoat. Stand in front of the mirror. Ensure the waistcoat is buttoned up and the waistcoat's front is overlapping the trousers' waist.
Grab the buckle's right side that has been inserted into the buckle. Pull the buckle's right side through the metal buckle until the vest fits snugly across your stomach, the buttoned front does not gape and you look slim.
To make the process easier, enlist a friend to adjust the waistcoat's belt while you wear it.