To calibrate your compass, move away from all magnetic sources (metal items) to avoid compromising accuracy.
Generally speaking, you shouldn't calibrate your compass indoors.
Position the watch horizontally. This is very important; calibrating the watch when it is not completely horizontal, may result in incorrect readings.
Follow the instructions in the product's user guide and/or those on watch screen, in CALIBRATION mode, and slowly rotate compass (360° in minimum of 10 seconds). You may need to rotate the compass several times to calibrate it properly.