You need to plot it when you need to add weight because the CG is outside the forward limit. Use the current weight and moment for one point, then find the weight and moment once you have added the 100kg or whatever further aft and plot this. Once you have plotted both points, join the dots, and where it intersects the moment envelope is the weight to add.
Bob uses the flow chart for the for adding weight (or shifting weight) when the CG is outside the aft limit. I never really understood the flow chart, so I just added the weights and moments step by step, then used the formula to work out weight to add/shift. It's a bit hard to explain it any better than this in writing.
Once you can do these questions in the review section at the back of the book, you'll be in a good position to do the exam.