have downloaded the manual from the Maclaren site (here) and it says:
REMOVING AND REFITTING THE SEAT COVER
Remove hood from stroller if fitted.
A Twist shoulder strap end buckles and feed through holes.
B Feed waist and crotch straps through holes.
C Unclip seat cover retaining straps.
D Unfasten seat fabric from handle tubes and seat base.
E Slide fabric from seat back and base.
F When refitting seat fabric, insert seat back support tubes into fabric slits.
G When refitting seat fabric to seat base, loop fabric flap over legrest strap
and pass crotch strap buckle through both holes in seat fabric and then
hole in seat base.
H When refitting shoulder straps, twist buckles and feed through holes.
there are useful pics in the PDF too