All the advises above are very good to follow.
I just want to add a few small technical things and a few pictures.
First, as it was already said, you shall need a straight edge that can be anything from an expensive aluminum or just a wide (4"~6") Plywood, MDF or Masonite.
Make yourself 2 "Off-set plates". how? I don't know how
but look at this post...very simple
When you measure, use always the same measuring tape or rule.
If you have seen a bullet hole on a concrete wall or alike, the side that the bullet entered has very "clean" hole but at the outlet side, the hole is chipped-out and larger.
That's exactly what happens when the blade teeth are attacking the wood...the teeth "entry side" has a clean cut but the teeth "outlet side" is chipped out.
When you are cutting with circular saw it means that the lower (under) side of the board will have a clean cut and the upper side will be chipped out.
If you don't care how one side will look, just keep the side that will be visible (the good side) under but if you want both sides to have a "clean" cut....
You can use the "Scoring blade" method....set your blade to around 1/16" below the base plate and make a cut (a score) backward i.e. starting from the far side and pulling the saw backward. This will leave a clean cut (score) on the upper surface of the plywood.
Now, set the blade to cut the full thickness of the plywood and cut the normal way forward. If your blade is at 90° to the base, it should leave nice and clean cut on both sides.
And now to the pics...