Peel and cut into batons. This can be done the night before, in which case store them in water.
Roast, stopping half-way through to flip the veggies and turn the tray around (the oven is hotter at the back).
You're aiming for "leathery", rather than "crispy". That's not a very appetising way of describing it, but they lose their sweet flavour if you char them. Save the charring for broccoli and sprouts.