NO Spray!!!! Just till your plot, pull the weeds, prepare your plot by raking it off level and compacting it, then sow your grass seed, sprinkling it with a little mulch or compost over top.
Then thoroughly water it, cover it with plastic and weight your plastic so that it doesn't blow away. Check the dampness of the soil every day to ensure that it is staying damp under the plastic, and if not, give it more water.
Once your grass has popped up, remove the plastic so your new tender grass won't get sunburnt.
Don't mow your new grass right away until is has gotten really established as your mower will just pull the new plants right out of the ground.
Then sit back and enjoy!