Break down large lumps of soil, then use a fork to roughly level the area. On heavy clay, add 1-2cm of coarse sand.
Shuffle back and forth across the surface, using your heels to firm the soil. Use a rake to break it down to a finer tilth. Check it's level.
Spring and autumn are the ideal time of year to make a new lawn. The method is the same for sowing seed and laying turf.
Repeat step 3, shuffling at right angles to the original firming. Rake again, holding the rake low to the ground to make it easier to create a level surface.