Attaching:
First, ensure two of the stroller hook straps (velcro + metal carabiners) are attached to the top two "D" rings on your Stroller Sling.
Open up the velcro and tightly loop each one around the top most portion of your stroller's handlebar. (The exact position can vary based on stroller-type. If you need more slack you can often lower the hook straps but scooting them to the sides of the handlebar and a bit lower down the stroller frame.)
Now attach the extra 2 carabiners to the bottom D rings.
Finally, attach the last two stroller hook straps (velcro + metal carabiners) to one of the lowest and front-most positions on the stroller by looping the velcro tightly as you did with the top two.