The best approach to adjusting the compass is to work in conjunction with a marine chart. Choose some points that you can align by eye that are within 10⁰ of North/South. Run your boat along this line and adjust the port/starboard adjustor on your compass accordingly so it reads the same as the chart (following manufacturer’s guidelines for adjustment).
Once the compass is running true to this line; test it again on an East/West bearing; and adjust the fore/aft adjustor until it’s reading correctly. Next, run the boat back down the North/South line (the opposite direction to your initial course). If the compass is off at this point, there is an error in mounting to the craft, and the compass must be re-aligned before completing the process again.