For a lot of channels you will have the option to map variant attributes, by default we will map attributes based on name e.g. where you have a size attribute on channel when we download it we will map it to a variant attribute named size in StoreFeeder. However you can override the default mappings and add your own.
Below is what the mapping looks like, this screen is with the default mappings (most customers can use just the default mappings):
Creating your own Mappings
You may find however you do want to map your attributes up a bit differently to the default mappings, to do this you just click on the variant attribute set (this column has the variant options from channel) and then under Variant attribute (these are the StoreFeeder attributes) you just select what you want to map the option to.
Once you have selected your custom mappings, these will replace the default mapping that were there previously. As an example, the picture below shows that Size is mapped to Item Packaging Quantity on channel.