There are two ways to cut down your handlebars to make them narrower. You can grab a hacksaw and go to town on them or you can get a pipe cutter and get an even and precise cut.

Pipe cutter method
Before you begin make a mark on both ends of your bars at the point that you want to cut them off at. You want to make sure that you cut the same amount off of each end or you'll end up with lop-sided handlebars which is no good.
To cut your handlebars with a pipe cutter, attach the pipe cutter to the bars by tightening the large screw at the bottom until the bars rest between the roller mechanism in the pipe cutter and the blade. Now, turn the pipe cutter in a circle around the bars repeatedly. You need to tighten the pipe cutter gradually as you go, and I do mean gradually. This process is going to take a while but when you're done you'll have an even cut with no jagged pieces of metal sticking out to make mince meat out of your hands. If you have the time I definitely recommend this method. My only suggestion would be to take your bars off of your bike and get a couple of videos to watch because it does take a while.

mark handlebar on both sides

Hacksaw method
This method is much quicker but the outcome isn't as good. It's rare that you get a straight cut all the way through the bars and you always have to file down the end after your done. Hacksaws are inexpensive and so are the blades. You can get a good hacksaw for under $5 and blades usually run about a buck a piece.

When you begin you'll want to take it easy and get a good groove cut into your handlebars. Try to keep the blade as straight as possible the whole time. Once you cut a decent groove into your bars you can start sawing as fast as possible until you've cut it all the way.

two methods shown

So what did we learn from this little tech tip? Well, we know that the hacksaw works the fastest when you have good blades, but it isn't the best way to do it. We also know that the pipe cutter does the best job, but you should get the $25 one, an extra blade, and have some time on your hands. Finally, we have learned that as a combination the pipe cutter and the hacksaw do a good and quick job at cutting your handlebars. (Source bmxonline.com)