Hmmm, I’m not sure I have a system as I tend to rely on what appeals to my eyes, but maybe there are a few things I think about. I use a bead board and start with what I want to use for the center pearl. Then I choose some of the pearls that I want to draw the eye to and place them along the sides around where they would hit by the collar bone or just below if it’s a short strand. If it’s going to be an opera or rope length, then I place them lower than that, but still higher than halfway down the rope. Then I start filling in from there, thinking about contrasting colors next to each other and random sizes. I really like randomness, so I typically don’t create any sort of pattern with the sizes. Or maybe it’s because I don’t have exact numbers of pearls in each size to create a pattern. I like the mix of colors, sizes and shapes because I think it draws the eyes all around. I hope this helps. I’ll have to think about what I do the next time I’m putting together a strand.