To find the reference angle you need to identify first the quadrant of the terminal side.
First quadrant: angles from 0º to 90º
Second quadrant: angles from 90º to 180º
Third quadrant: angles from 180º to 270º
Fourth quadrant: angles from 270º to 360º
Use the next rules to identify the reference angle in each quadrat:
1st quadrant: reference angle is the same angle
2nd quadrant: subtract the given angle from 180º
3rd quadrant: subtract 180º from the given angle
4th quadrant: subtract the given angle from 360º