Sharp blades, cut slow, cut from the back side. Bewfore flipping it over place a tape along the space where it will be cut to help prevent splintering.

If you have a belt sander, cut an either inch long and then use the belt sander to bring it true to the line.

Hey Al,
There are tools called scoring knives that have a carbide edge and can be used before any cut is made. Just score a line where you'll be cutting, with the knife, all the way down to the particle board or whatever the top is made of and then cut right up to this line without any chipping from your saw. It might not hurt to practice with a piece of scrap. You should be able to cut through the back with a good sharp cross-cut handsaw. It really won't be that hard. The top itself can be cut using a circular saw or saber saw and then touch up with your sander.Good luck......MJH

No scoring isn't going to help you here, that's for applying new sheets.
Do as Pif says but I make the cut within an 1/8 and belt back.. it's the trades standard way of cutting c/t.
Even to get to the scribe at the wall same process but from the top of course and with a slight angle, then belt.

What was I thinking.....MJH

if you can take the top off, just flip it upside down and cut it. I'd suggest a nice router and bit...if not you can alwas attach/clamp a straight edge and use a rotozip tool ( basically a router)... but a belt sander after you cut it is gonna make the finished product alot mor finished whatever you do.