Choose any of the diagrams in the paper, draw a vector from the center of the diagram (the origin) to one of the particles, then draw another vector from the origin to a different particle. Add these two vectors (SGB: move the base of one vector from the origin to the non-origin end of the other while keeping its orientation the same), and see if the result lands on a third particle -- if it does, that's the result of the interaction.

--
simon