To adjust the harness straps on a Britax car seat, refer to the car seat and vehicle manuals for specific guidelines. Locate the harness adjuster strap at the back of the seat. Pull the strap and slide the harness straps up or down to fit the child snugly. The straps should be tight enough to prevent movement, but not so tight as to restrict breathing. Secure the straps by inserting the excess strap into the adjuster slot. Ensure the chest clip is fastened at armpit level, and the crotch buckle is positioned between the child’s legs.