If you have a plunge saw with tracks cut from the top but make sure you set up the cut so the splinter guard is on the side you want to keep. You'll get better visibility that way and will be less likely to mess up all your hard work.
With pretty much anything else cut from the bottom so that the leading edge of the blade is pulling into the surface. But remember to measuremeasurecut.
It depends on what saw you are going to use.
If you have a plunge saw with tracks cut from the top but make sure you set up the cut so the splinter guard is on the side you want to keep. You'll get better visibility that way and will be less likely to mess up all your hard work.
With pretty much anything else cut from the bottom so that the leading edge of the blade is pulling into the surface. But remember to measuremeasurecut.
This belt sander is under £50 if that helps.