Take the clones when it suits your needs - whatever happens, it will take at least 3 weeks before you see a vegetative clone take roots and start to grow a little. However, once they start, they grow rapidly.
As someone else has already alluded to, you can take clones once the flowering phase has started, but this will require: close to a 24-hour light schedule to revert them to veg; hugely extended rooting time (2-3 times longer than with a veg mother); more care and attention as it will stress the plant massively and probably have a higher clone failure rate. That said, it's very possible and I have done it myself with a plant that was 2-3 weeks into flower.
Step-by-step advice for your first cuttings:
1. Use a clean pair of scissors and cut diagonally on the stem.
2. Remove everything but the top most set of leaves and growth on the cutting, and then cut the tips off each of the two remaining leaves so that transpiration is reduced.
3. Dip the new clone into rooting powder or gel (both work well, but I prefer the powder to help dry out the cutting end and prevent mould/rot) and stick this straight into a pre-soaked Root Riot sponge or Rockwool cube - I prefer Root Riot over Rockwool for cuttings personally. I pre-soak mine in a weak solution of Rhizotonic (a rooting stimulant).
4. Place in a propagator, and mist the plant with water - again I sometimes add in some Rhizotonic as a foliar feed here. You will need to mist the plants every day to keep the humidity high inside the dome. Plants will absorb their water through the leaves until roots take hold, so they needed to be dampened regularly. Do not use freezing cold tap water!
5. Leave the cuttings to work their magic at temperatures between 20-25°C and out of direct lighting - once roots appear you can put them under lights and start to water normally.
Good luck!