I wonder about where your instructions say to "alternate" strips from the two sets.
For me alternate is a word you use when you have two sets which are all the same ("alternate blue and white strips" ). I think that the operation you are describing is more complex.
Do you mean
Cut each set as shown, to give 2 sets of vertical strips. Do not move the strips from their places after cutting, but keep each one carefully in its place. Starting from the left hand side of each set, take one strip from each and place them on a suitable separate surface, building up the design by adding a strip from each row in turn, until you have used all the strips from each set in order.