Hi there, welcome to the forum.
Old and cheaper kites often come with the line wound on 'yoyo' spools or handles and you just unwrap as much as you want - problem with this is that the line lengths aren't always the same and that can stop the kite flying properly.
The HQ ready to fly kites ( and most other kites) come with the lines pre measured on a winder so both are the same length. You attach the loops at one end to the tow points on your kites bridle, unwind all of the line on the winder and then attach the wrist straps to the other end - make sure you have them in the right hands , if you attach red loop to the right side of your kite make sure you have the handle with the red loop in your right hand.
This video explains the set up - it's for the Limbo but will be the same for your Stratus.https://www.youtube.com/watch?v=XfuOAnzxF3k