In order to assign simple products to the configurable ones, it is necessary for them to already exist in Magento. Therefore, at first, I'd recommend importing all the products.
Here are the minimum required fields to import a new product:
Root Entity
For more convenience, please, feel free to add product attributes such as name, price, description, etc.:
Here is an example of the import file:
Then, in a separate import profile, please, enable these fields:
Root Entity
Configurable Product Super Attribute Entity
Configurable Product Super Attribute Link Entity
The product_id and parent_id are the IDs of the configurable product, which may be checked in Catalog > Products.
Here is an example of the sample file: