1) Create the option attribute and values.

2) From the product grid find a child product, edit and assign the correct value from your attribute. If not already ensure visibility for this item is set to 'Not visible individually'.

3) Repeat the above step until all your child items have the attributes assigned.

4) Create the configurable item and add the configurable options (create configurations). 

-Click on the Attribute that is assigned to your child items.

-Follow through to next

-Choose 'one' value (this is to create a temporary dummy item)

-Follow through to next (no action)

-Follow through to generate products

5) Now remove the item you created by using the drop-down

6) Click on the blue link 'Add products Manually' that has now appeared.

7) If required use the filter to find the child items by sku or name, otherwise, simply tick the box (left-hand side) for your selected child items and click done.

8) Items are now assigned - do remember to save the product!