Flat handlebars are usually sold in a one-side-fits-all format so new ones are too wide for almost all riders. For most applications the handlebar width doesn' need to be wider than the rider's shoulders.
Move The Controls
If you’re not sure about trimming your handlebar width, move the controls inwards and spend some time riding in that position before you cut. Once you cut there’s no going back.
Measure Handlebar Width
Make sure the handlebars are centred in their clamp. That exactly the same distance sticks out on each side. When you’ve decided how much you want to trim off the handlebar width, mark the cut-line by scratching the bars with a file. Measure the amount to be cut off and scratch a line on the other side with a file.
Position a rag to gather the metal dust and make sure it won’t get into moving parts. Begin the cut by dragging the hack-saw blade toward you a few times.
Cut Handlebar Width Carefully
Saw gently keeping the blade square to the handlebars.
Once you have finished cutting the handlebar width, use a file to remove any burrs. Repeat on the other side.