2012 Toyota Sienna Owners Manual - Page 237
2012 Toyota Sienna Manual
Page 237 highlights
1-7. Safety information STEP 4 While pushing the child restraint system down into the rear seat, allow the shoulder belt to retract until the child seat is securely in place. After the shoulder belt has retracted to a point where there is no slack in the belt, pull the belt to check that it cannot be extended.