What's the easiest way to become a good guitarist in a rock band?

Easiest? I'm not sure there is an "easiest" way, but practice is the only way. And the only way to get better by practicing is to practice what you aren't good at. Guitar players tend to turn practice by playing what they are already good at. It's fun to play what you like doing, but practicing things you find difficult is the best way to get better. In other words, performing is where you sound good, practice is where you want to sound bad.

If your main criterion is easy then you're probably not going to make it. It takes a lot of hard work, time, and practice.

Learn, properly, how to play guitar. Then get a few years experience, join a rock band and get a few more years experience in that bad .

Be in a band and practice your butt off. Nothing teaches you to be better than being in the mix with other musicians. A bedroom cannot give you the same experience.

Make sure to get a really talented drummer and a good singer. Let them shine and just rely mostly on effects for the guitar parts. The guy from Limp Bizkit does this. He isn't ripping solos like Kirk Hammett but hes using mostly delay effects to create his guitar parts. And that works for him.

Make simple music like Status Quo.

Practice half of forever and then practice some more

There is no ready way. Hard work and dedication. If your looking for the easy way it may not be the right thing for you.

To practice and form a band

Be in a pre-fab group.and let a pro play the hard parts.