opnspaces writes "I would go with the router too, although I have limited experience with a router so I'm not sure how well it would come out"
You can clamp a straight board across the plywood to use as a straight edge. The straight edge is placed so the router base rides the straight edge. You measure from the bit cutting edge to the router base edge, that number is your "offset" that you would add to how far in from the edge the straight edge is placed.
I use the same idea of clamping a straight edge to boards when making long cuts with a circular saw or even a jig saw. Makes for real nice straight cuts.
Alternately you can buy bits which have a bearing that rides the edge of the board. this works great as long as you don't need a real long cut into the board edge.
Routers can get away from you if you are not careful but they can be rather useful at times.
Another way to do this is if you have a table saw with a dado blade, but can be a bit difficult to do when dealing with say a 4x8 sheet of plywood..