The preload is set by adding/removing carrier bearing shims. The process is explained pretty well in any of the service manuals for the IH-built Cub Cadets. The only thing wrong is that the explanation of where to move the shims to get the right contact pattern is bassackwards. The slightly tricky part comes when you get the preload right and then have to move the carrier left or right by moving carrier bearing shims from one side to the other. The good news is it's pretty easy to decide if you've got it right or not by measuring the backlash with a dial indicator and making sure it's on the low end of the spec.
I would highly recommend reading the transaxle disassembly/assembly instructions in both the
CCC service manual and an
IH service manual. The IH manual is written better but both are useful.
Read those and keep asking questions until you know what you have to do.