Soil quality and preparation are important steps in planting a new lawn. Begin by removing the existing vegetation. The weeds must not be moisture stressed when they are sprayed. Let Round-Up dry on leaves for 12 hours. Dig into the soil and get a feel for the soil particles.
Sandy soils drain quickly, and have good root growth.
The clay soils tend to drain poorly and become compacted. It's much easier to add fine soils to coarse sandy-soils, than to add large amounts of sand to dense clay soils. Examine the soil and overall suitability of the site. If so, these should be removed. A soil test should be done in order to add proper fertilizer types and amounts.