Most lawn specialists and horticulturalists recommend clearing the existing vegetation from an area where a new lawn is to be planted by spraying Roundup or another brand of non-selective herbicide that does not linger in the landscape. The active ingredient in Roundup is glyphosate. After the first application of Roundup, keep area moist for two weeks to encourage any lingering weed seeds to sprout and apply another application of Roundup. After another two weeks, the area should be safe to add seed for the new lawn.
How to Plant Grass Seed After Using Roundup
Rake the area where the Roundup was applied and remove all dead vegetation so the grass seed can have contact with the soil when it is spread.
Mix seed with topsoil at a rate of one part seed to three parts topsoil and put in a bucket so you can broadcast the seed by hand and it will mix with the topsoil. You can also use a mechanical spreader set at the level recommended for the type of seed you are spreading. Be sure the soil is dry so the seed will not stick to your shoes.
Spread a thin layer of topsoil over as much seed as possible after the seed is spread over the lawn. Step on the seed to press it into the soil.
Water the seed with a gentle spray of water every day until green appears and then every other day until established. After established, water the lawn at the equivalent of one inch of rainfall per week.