Can you suggest a novice method for detrending and deseasoning time series to find relationship between two variables? I have a time series with two variables: discount and sales. My goal is to figure out how the discount affects the sales. I want to perform regression. However, the sales data has a trend and seasonality. What could you suggest for removing the trend and the seasonality given that I'm a complete novice? First and foremost, my goal is to learn.
For clarity, I plotted sales and discount (normalized) in time:


I would recommend looking at Holt-Winters seasonality (explained here).
It decomposes the time series in to three terms: level, trend and seasonality.
