Mix in either watering can or sprayer, spray, watch the moss die before your eyes, but then be prepared to either rake or scarify the lawn, fertilise and watch it green before your eyes!
Just ask for sulphate of iron at any hardware store or garden centre. I just mix it with sand and spread it on infrected area. It will turn black in a few days and will look ugly. I do not bother raking it (too much bother) just let it die. It will look good in about 2 weeks
One of the main causes of moss in lawns is cutting the grass too short; if this is the case with your lawn, then treat the lawn as described above but also reduce the depth of cut.