This always trips me up so I'm writing it down, I hope this commits it to memory.

AttributeDefinitions = KeySchema + Indexes

So if the table has a KeySchema of {KeyType: "HASH", AttributeName: "my_primarykey"} and an global secondary index named {KeyType: HASH, AttributeName: "my_index"} then you simple add these into the AttributeDefinition and give their type. Like the following.

AttributeDefinitions:
    - AttributeName: "my_primarykey"
      AttributeType: S
    - AttributeName: "my_index"
      AttributeType: S

Very simple. It says this pretty much verbatim in the docks and yet I always spend five minutes futzing with it. No more!