Action Details
Requirements and how-to articles for Amazon's action:

Encourage customers to buy multiple products by offering a "buy one, get one" promotion.

You need this to get it done:

Buyer Purchases
The purchase requirements for the promotion to apply. This can be either the minimum quantity of items purchased, amount spent or for every quantity of items purchased.
Purchased Items
Choose a product selection for the promotion to apply to, or create a new one. You can also select your entire catalogue.
Product Selection Name/Tracking ID
If you are creating a new product selection for this promotion, you must give it a name.
Internal Product Selection Description
If you are creating a new product selection for this promotion, you must provide an internal description.
If you are creating a new product selection for this promotion, you must add a list of ASINs (Amazon Standard Identification Number) to indicate which products you are adding to this selection. You can add up to 4000 individual ASINs to a product selection.
Buyer Gets
This field is currently limited to only have "Free Items" as an option.
Applies To
By default this is set to purchased items.
Item Quantity
Indicate how many free items the customer should receive for each purchase.
Excluded Items
If you wish, you can exclude products from your promotion. Choose a product selection, or create a new one.
Set the start and end dates for your promotion. Note that the start date must be at least 4 hours ahead of when you submit your promotion.
Internal Description
The internal description for your promotion.
Tracking ID
The internal name/tracking ID for your promotion.
Claim Code (optional)
You can choose to create a claim code which customers must enter at the checkout in order to receive your promotion. There are two different types of claim code which you can create: single-use codes (can only be used once) and group codes (one code that can be used an unlimited amount of times until the promotion ends.)
Claim Code Combinability
There are three options available for Claim Code Combinability:
Preferential: Only one promotion will be applied to the purchase (if the customer attempts to add more than one promotion, only the one with the greatest discount will be applied)
Unrestricted: This option allows multiple promotions to be stacked (be very careful about choosing this option!)
Exclusive: No other promotions will be able to be applied at the same time as this promotion (no stacking)
Checkout Display Text
The message shown to the customer at the checkout.
Detail Page Display Text (checkbox)
Check this box if you want the promotion to be displayed on the detail page.
Purchased Items Display Text
The name of the items in the promotion. This is used in the detail page display text.
Detail Page Display Text
The text that is displayed underneath the “Promotions and Special Offers” section of the product detail page.
Requirements missing or found errors? Report

How-To Articles
A list of useful links for you:

Amazon Seller Central - Promotions
Information about Promotions.
Link missing here or found errors? Improve How-Tos