Developing the Board
Prepare the board resist developer solution using 10g of pure NaOH granules (meter using analytic balance) per liter of water (about 250ml in the small tray more than suffices). Room-ish temperature is sufficient, hot is bad (accelerates development and punchthrough, potentially just stripping all resist from the board).
Holding the board in a gloved hand, immerse and gently swish/agitate it. You should promptly see green streaming/boiling/clouding off the board and the emergence of the artwork pattern. Inspect closely every 30 seconds or so - the exposed copper to be etched should be bright and shiny. Any hint of gumminess means that there is a thin film of resist still clinging to it.
If the developer is fresh, this gumminess means that the board was underexposed. There's nothing to lose at this point by keeping going in development to see if it will clear. It will not be too very long before the caustic will begin to strip the unexposed resist as well, however.
Once done, rinse both sides under fast running cold water to get the caustic off. Gently DAB dry using a paper towl. DO NOT rub: The resist isn't a delicate flower but it's not that tough either.