While this may not always work, I usually add 2 to the number of guides suggested for the rod lengths indicated in the table above. For example, a 7 ' casting rod for crankbaits listed above would use 7 guides from 16 mm down to 8 mm rings at the tip. I would probably add another 10 mm and a 7 mm at the tip or another 8 mm if I couldn't get a 7. Some of the choices you make become second nature after you've built a few poles and start getting comfortable with what you are dealing with. Guides are classified by frame style, ring material, color, and ring size.
Ring size is usually the last number in a description of a guide.
It took me a long time to get a handle on what information guides labels were providing. I'm going to let you in on two secrets that will help you figure out what you need. For your first few rods, buy guide sets from your suppliers. Cabela's and Dale Clemens both offer complete guides sets without tip tops based on the table above. Ring sizes are a bit different with Cabela's but both companies offer complete sets.