...just realized what might seem self-evident might in fact be somewhat obscure to some so I'll elaborate...
Rig a simple flashing source near the lens of the enlarger (a clamp on fixture with a 15w bulb and some 216 white diffusion works very nicely). Control the duration of the flash by connecting this fixture to a timer, preferably with a foot switch. Determine your flash time. Cover the enlarger lens with a red filter and use a burning-in card (white side towards the lens, black side down) to do your selective flash. It's a simple technique that works well when done properly.