At the end of the day it’s rng.
For quite some time the commander at al‘merayah gave so much experience that we could instantly level our followers to lvl 20.
@Wak4863 made some videos about this:
I also recommend this thread for very detailed information about followers:
However, i prefer to level them “slowly”.
I completely fill the inventory of my followers with the food that increases the growth chance in the desired skill. (Simply because i‘m too lazy to check their xp-bar and food in their inventory all the time)
Then I start fighting enemies which are strong enough to deal damage to them (to make sure they eat their food), but not strong enough to kill them.
Wolves, bears, tigers, rhinos and stuff like that.
Depending on their gear i start fighting world bosses with them when they reached lvl 5 to 6.
If their gear is crab it’s better to wait until they reached a higher level, 10 or more.
But it’s still a lot of rng involved. 
In the current state, most followers are useless anyway, especially given the fact that they take fall damage now! 