As simple as it gets - LED bike lamp, tray, glass, water and washing up liquid.
Or - LED bike lamp, plastic sandwhich container, jam jar, water and washing up liquid.
Put the glass in the tray, rest the lamp on top of it, add water with a touch of washing up liquid to the tray.
Leave overnight. I find that different areas of the room get different numbers of fleas.
You might notice lots of fleas on one side of the tray - presumably they've all come bouncing over from one side of the room.
Haven't tried the flashing light setting yet.