Calculate what the hypothetical constant demand was in each quarter i.e403/4. Calculate in what ratio the actual demand in quarter was above or below the constant demand i.e80/100.75. Decompose data into the trend component i.e80/0.89 and seasonal component i.e80−80/0.89. If you have a monthly data consider converting the 12 data points per year to 4 data points with a weighted average. Basically taking the average of every three month to compute one data point. Weighing the start and end of season less than the middle of the season.