How do you make a time-series forecast method?

I have to do a project on Meriwell Bag company and one of the questions is asking for me to develop and justify a forecasting method for the companies specifications. From what I read of their specifications which include a desire for stability as that is what their company and customer base has been, want to track the number of bags sold during the holidays so there is a seasonal factor here, anticipate future growth of customers and they want a method that works with their aggregated demand. I read back in my textbook over several types of forecasting methods and fond the time-series method, however my book only goes as far as to show and talk about the decomposition of time-series data over time. So I was wondering if anyone had a clue as how to go about making a chart or something for the times-series method. I'm sure I can justify why this method is the best. I just need to know how to make the chart or whatever is required to develop it. I did email my teacher, but no answer yet.