You should receive an email from Hemifever after you purchase the custom tunes and a programmer.
When you get that email it will have instructions on how to use the programmer to log your truck's performance. Those logs will capture how your truck performs with the mods you have on it.
Once you are done logging you download those logs to your PC and email them to Hemifever.
Hemifever will use those logs to create a custom tune built for your truck. Then that tune file will be emailed to you.
You will add that tune file to the programmer then upload it to your truck.
Then you will log your truck's performance once again, download the logs to your PC, email them to Hemifever and he may tweak the tune a bit more, then send you a new tune file.
Load the new tune file to the programmer and then to your truck.