Attachment 58292

Attachment 58293

Attachment 58294Next I use a saw refered to in US as a 'bow saw'. This one costs $12.50 US at our version of 'Bunnings' , and it's cutting edge measures 24". Holding it firmly against the straight edge begin cutting straight down until a good kerf(?) is started.
Next I like to tilt the saw and cut the corners, as the saw blade has a tendency to veer to one side at the corners if you cut all the way straight down.