Before you perform a perc test, choose the right location for your septic tank. Determine where your outlet pipe will be and where your drainage field is likely to start. Dig a 300 mm x 300 mm hole below the proposed invert level of the outlet pipe. Excavate a larger area where the invert level is quite deep. Remove any loose debris. Fill the 300 mm section of the test hole with water and allow it to seep away overnight.