Either shingling a wall or a roof, there is no magic formula, just practice and experience. I will give you a couple of hints. (trade secrets so don't say I told you!) Since you are working on a roof, this can be a little tricky but doable.
First, separate your shingles into piles by size. You can usually make 4 or 5 different piles. Open 3 or more bundles at a time so you have lots of shingles in the sorted piles. You will notice that there will be lots of the medium sized ones, smaller ones and less of the largest and very small ones. Until you get the feeling, you can lay out maybe 15 to 20 shingles on the row, take a look to see if that is the random effect you like, adjust a bit, etc. Be sure to work in more of the sizes you have the most of so you won't end up with a bunch of small or medium ones left after all the largest and smallest are gone. Now you can go ahead and nail down that row. Repeat this process, keeping a good supply of sorted shingles to chose from. Be absolutely sure to bridge all the joints of each row with the row above. I think that after you have done a few rows, you will start getting a better feel for what you are doing and the process will go faster.
If you have someone to work with, having one person laying out shingles and playing a bit with the sizing, and another nailing also makes the job easier.
Be careful up there as well. Good luck
