Hello Alan; This is currently being discussed and if you read the linked discussion and follow the links within it, you may get enough information. I have some plywood images attached to that discussion. My apologies if it is too much information but the background stuff is also useful.
What type of things are you trying to engrave onto plywood? The laser should be capable of burning an image into plywood. I am guessing that you are using an original Snapmaker machine with a 200mW laser beam.