How to use the minecraft 1.9 "Passengers" tag?

Since Passengers is a list, you separate each rider (the slime) of the root entity (the chicken) with a comma. This differs from Riding in that each rider is not required to ride one another, though it is still possible by including Passengers with each rider.

The following would summon a pair of leather boots as the root entity, that had 3 other riders at the same depth:

/summon Item ~ ~1 ~ {Item:{id:"minecraft:leather_boots",Count:1},Passengers:[{id:"Item",Item:{id:"minecraft:leather_leggings",Count:1}},{id:"Item",Item:{id:"minecraft:leather_chestplate",Count:1}},{id:"Item",Item:{id:"minecraft:leather_helmet",Count:1}}]}

If you want them nested further, similar to a Riding stack, you'll have to add Passengers to the rider:

/summon Item ~ ~1 ~ {Item:{id:"minecraft:leather_boots",Count:1},Passengers:[{id:"Item",Item:{id:"minecraft:leather_leggings",Count:1},Passengers:[{id:"Item",Item:{id:"minecraft:leather_chestplate",Count:1},Passengers:[{id:"Item",Item:{id:"minecraft:leather_helmet",Count:1}}]}]}]}

I suggest you to use McStacker selecting as Minecraft version 1.9 Snapshots