Release the velcro, press button, pull seat until it clicks.
Unfold the cover to release seat belt positioners, bring cover back up and fasten positioners with Velcro tabs.
Tread the lap belt under both red positioners and across the hips of your child. Add the shoulder strap to the back of the booster and the diagonal seatbelt, up the back of the child at their shoulder tip. Add the Impact cushion to the diagonal seatbelt, slide it low down to the child's hip bone beside the red positioner.