Probably the easiest method is scoring heavily with a box knife along a straight edge and snapping over an edge.
A large sheet though needs a different approach. I used a plunge saw
with a clamped pine length as a guide.
Aligloss White/Black Composite Panel - 1220mm x 810mm x 3mm
with two transverse cuts down the middle gives me four 24x16 inch panels for $36.50 or $9.13 each.
A bit of a beast to drive, but cuts were clean, smooth and fast.