It's ok to take clones early in to flower. Normally the first 2-3 weeeks at the latest. Take them from the bottom of the plant. They should grow roots in the normal time. Try using the DIY bubbleponic cloner in the DIY section of the FAQ. There's a 20 inch tupperware container that a propagation dome fits perfect on. I've had roots a few days in cuttings taken from plants 2.5 weeks into flower. Used the bubbleponic method with a dome and it worked great.
It's easy to screw up and end up with them stretched or deformed, but veg long enough and they will be fine....