Lawn 'feed, weed and moss killer' should be applied between April and September. Do not apply during prolonged dry periods or droughts. Apply on a warm still day, when the soil is moist but the grass is dry. Try to stay off the treated area until it has rained or has been watered in. Water the grass thoroughly if no rain falls within 2 days of application, to wash in the weed and feed.
Don't cut the grass for 2-3 days before and after applying
Any moss in the lawn should turn black after a few days, and is to be expected. Rake this out, with a sprung, tined, or bamboo rake after a couple of weeks. This scarifying will also encourage new grass growth. Treated grass clippings can be added to the compost heap.
Don't apply fertiliser or weed and feed to lawns that are less than 6 months old. You'll put undue stress on the new grass.