After the exposure, the board should be immersed into the development solution immediately.
This solution is made of 10gr NaOH in 1 L water (you can do a lot of pcb's with one liter).
Put the solution into a plastic or glass tray and immerse the board in it for about 1 min.
Wear gloves to protect your skin and safety goggles!
Shake the board around a bit and after a minute in the solution a faint 'picture' of your layout should appear. Quickly rinse your board with clean cold water to stop the development proces.