Your goal is certainly achievable, and it is actually a very modest gain. Most people try to gain much much more. As DLD stated, start with the
newbie routine. Stretch as much as you can throughout the day on top of what the
newbie routine recommends. When you wake up stretch, when you go pee stretch, when you change clothes stretch, etc. Also, a lot of length gains for many people come from jelqing, which is in the
newbie routine also. If you learn to properly jelq and do so for at least 20 minutes a day I guarantee you will eventually get to your target length.
All of that being said, I still think most individuals would be better off using some sort of device on top of any manual work. If you are focused on length, after you do the
newbie routine for a few months I suggest either getting into hanging or else buying a good
extender. These devices allow you to continue to stretch even when you aren't using your hands, so you get both more time under tension than you would manually and you also supplement and help cement any gains you would make with your hands. That is the basic idea behind
SRT, which I also recommend that you read up on.