Like the LP nightmare before it, Masonite faced a $40 billion class action suit against it for its siding products in 1995. It is my hope that the masonite you plan to use is not on the side of your home...
In any case, you may certainly use either a hand saw or a power saw. You will probably find a power saw to be easier to use. For blade selection, choose a combination blade with a high tooth count and use carbide tips for longer saw life and quality of cut. This type of blade will run about $12.
When you make the cut, make sure the blade goes into the face of the board (the face is the part you will see) so the blade doesn't accidently blow chunks of the surface off. It isn't a bad idea to score the cut line with a utility knife first as well.
When you apply your board, if you choose to screw it, be sure to pre-drill. This material is very dense and will likely split if you don't pre drill your holes.