There are several Chicco high chairs on the market. The the padded fabric seat covers are removable for washing. In all cases, the covers have slits for safety harness straps. When replacing the seat covers, you need to pass these straps through the slits. On some models -- like the Polly 2 in 1 -- the seat cover also fits over a rigid plastic crotch strap.
Place the cover on the seat of the high chair frame. Fit the top of the cover over the top edge of the seat. Pass one of the safety harness straps through the appropriate slit on the upper part of the cover. Do the same with the other safety harness strap, at the same height position.
Locate the D-ring slits near the cover’s rear fold. Pass the two D-rings through the slits. Slip the fabric crotch strap through the slit near the front of the cover.
Place the reducer cushion -- also referred to as a booster cushion -- on top of the lower cover, if applicable. The reducer cushion is useful for younger or smaller babies. Pass the two safety harness straps, the two D-rings and the crotch strap through their respective slits.
Complete the safety harness with the lap belt. Slide the lap belt through the D-Rings. Insert the crotch strap clips into the buckle. Fit the safety harness strap spring catches.