Reach level 75 with Ranger.
Currently there are 2 main ways to level jobs: 'Book burns' and 'Abyssea (Leech) parties'
Abyssea is the quickest of the 2.
These can be done at any level, though up till 10 it's usually easier to just solo.
Book burns use so called grounds tomes which can be found in dungeons.
On the Asura server (and I assume the other servers aswell) the following are used:
Level ~10 - ~30: Gusgen Mines
Level ~30 - ~60: Crawler's Nest
Level 60+: Bostaunieux Oubliette
The benefit of the grounds tome is that they give bonus Exp for killing a specific amount of specific types of mobs. This bonus Exp is not dependent on the amount of people in the party/alliance, so it often means 12-18 people killing easy to kill mobs and finishing the requirements quite fast.
Also no 'newer' expansions or add-ons are needed.
Abyssea (Leech) Parties
For Abyssea access you need to be level 30, have the Wings of the Goddess expansion and have the required add-on for the zone you are going to.
Unless you pay or have a party to boost you, below level ~75 you will also need an ingame currency called cruor, which is usually farmed at higher level and as such you will need a higher level job already.
Below level 75 you are usually a so-called 'key-whore'. This means buying forgotten keys with your cruor and opening boxes dropped by mobs killed by the rest of your party/alliance.
The boxes give exp, cruor and bonus time to stay in Abyssea, so that adds up aswell.
Exp is on a build up, but it will go real fast after a while.
I don't know many of the zones used for this except for Abyssea - La Theine. The mobs killed here are worms, so it is often called a worm party.