To build a leach field in clay soil, follow these steps for a large leach field or a septic mound system, which is a raised leach field. First, conduct a percolation test to assess the soil's drainage capacity. Next, excavate the designated area to create trenches or a mound. Install perforated pipes in the trenches or on top of the mound. Backfill with gravel to enhance drainage. Cover the pipes with geotextile fabric and then soil to prevent clogging. Regularly maintain the system to ensure proper functioning and avoid septic issues. Seek professional guidance for optimal results.