Put it under a desk lamp fitted with a 60watt incandescent bulb for about 10 minutes, have the bulb about 5-6 inches above the work and do not leave under too long or it will scorch, too close to the putty and it will melt slightly and dis form your work.
I have a block of wood on which I place a ceramic tile, my work is placed onto the tile and a lamp placed above, I use an egg timer to remind me to switch off lamp so I dont get distracted and burn the work.
Dont try this with Duro(greenstuff) though, it melts and bubbles into a mess.
Steve