Grass growing between cracks in sidewalks, between pavers on your patio or other undesirable places can be a big nuisance. You can kill this unwanted grass a variety of ways. When choosing a method, you should consider proximity of the grass to other plants--many substances (including organic ones) that kill grass also harm other plants. You may consider using mechanical, rather than chemical, methods to kill grass near or in flower beds, for example--unless you don't mind taking extra steps to protect the other plants.
How to Kill Unwanted Grass: The Spray Method
Purchase either distilled vinegar or an herbicide. Vinegar will kill grass if it is used full-strength, but you will need to use a lot of it on a large area. Vinegar is perfectly suitable if you are just spraying grass between sidewalks or pavers, for example. To kill an entire lawn, however, it's easier to use a commercial herbicide--both chemical and organic herbicides are available.
Choose a calm, clear day to spray the unwanted grass. Windy days mean the spray may be blown into places you don't want it to be blown. Rainy days mean the chemicals may be washed off before they are effective--so plan accordingly.
Put on rubber gloves and safety goggles. Pour vinegar (if applicable) in a clean spray bottle or mix an herbicide in a garden sprayer. Follow label directions carefully if you opt to use an herbicide. Place plastic sheeting over neighboring plants as a precaution, whether you use vinegar or an herbicide.
Spray the unwanted grass with the product, thoroughly saturating it. With either vinegar or an herbicide you should see results (dead or dying grass) the following day; sometimes, however, you may need to apply a product more than once.
How to Kill Unwanted Grass: The Newspaper Method
Spread newspaper (the black and white newsprint pages and not glossy pages) over the unwanted grass. According to Colorado State Extension you should apply a thickness of 10 sheets of the newsprint over the grass. This thickness is sufficient to smother the grass.
Spread wood chips over the tops of the newsprint to weigh it down and keep it from blowing away.
Use a garden hose to lightly wet the wood chips to keep them from blowing off the paper. According to Colorado Extension, the grass beneath the wood chips and paper will die and decompose within a few weeks.