T-squares are usually pretty accurate assuming if hasn't been abused. I'm assuming you are using a fresh 4x8 sheet of plywood/MDF. The corners/sides on those are usually extremely square, but, check them out with your T-square before you begin (I have gotten some where the angle was not perfect, rarely though).
Try to plan your cuts so you're using these as much of the corners/sides as possible. Mark a line where you want to cut on the panel and use your T-square (clamp it to the panel) as a guide fior your router. If you end up straying from the line (for whatever reason) stray outside the line. That way you can sand down to the line if you make any mistakes. Take is slow and easy and make sure someone is there to help support the larger pieces. Other than that, any mistakes can usually be corrected with Bondo/Wood Filler.