Birthright is definitely the easiest of the bunch and story wise, it's best to start there. Conquest is pretty hard, not mincing words even normal mode might be only slightly easier than sacred stones hard mode, but if you get used to playing FE with birthright which has a rather smooth difficulty curve, you should eventually be able to handle it. Story wise, end with Revelations which isn't too bad, but there's more out of combat management stuff since you have twice as much stuff to build and upgrade in your castle also some unit balance is bit borked but about 3/4 of the cast is usable. After conquest, it shouldn't be too hard.
On a side note, I really like how they changed obtaining skills in Fates from awakening, they made it more resource management esque (while also cutting down tremendously on grinding) like previous games and every playable unit has a unique skill to them alone, which is neat.