How can I make a POWER-LED Lamp?
Ten in series will require 32V-34VDC at 300-350mA, or worst case just about 12W. You'll need a stepdown transformer from 240VAC to 36VAC at 350mA, and you'll need a half-wave rectifier (do NOT use a full-wave rectifier unless you use a 24V transformer!) good for 50V @ 350mA, and you'll need one stabilizing power resistor capable of dropping about 2V at 350mA (about 5.7 ohms), rated for 12W.

I mentioned a full-wave rectifier. You COULD use a full bridge rectifier with a 24VAC transformer secondary - that'd get you 24*1.414!=34V, but then you wouldn't have any room for the stabilizing power resistor so you'd be running on blind faith unless you removed ONE of those LEDs from the circuit and replaced it with an equivalent power resistor.

Do remember... this whole assembly will generate a significant amount of heat, and you have to get rid of that heat somehow or you'll overheat the LEDs and the resistor.