man bash. read, digest, apply, repeat. you can also look at existing scripts, figure out what they do, why, make copy, alter, see the changes in behavior. there are also books on bash.
larger point: how do you learn more about X? or get better at doing X? figure that general pattern out and you can re-apply it for anything, not just bash.
larger point: how do you learn more about X? or get better at doing X? figure that general pattern out and you can re-apply it for anything, not just bash.