First Off, you are going to need an Amazon Associates Account and some basic info on what you are doing inside.
Check this out for more info about Amazon's Affiliate Program!
Okay, now that you are signed into your Amazon Associates Account, in the menu you will see "Product Linking" , Hover over that to get your drop down menu where you will see "Native Shopping Ads" , Click Native Shopping Ads and you will be brought to a new page that should give you some extra info on Native Ads in general if you are interested.
Now that you are on The Native Shopping Ads Page, you should see at the top of the screen in the middle: "Recommended Ads" "Search Ads" "Custom Ads"
Go ahead and click "Custom Ads", you will be brought to a new page where the magic will begin!
Lets Get Into The Magic!