Sorry for the late reply. I didn`t get any notification about your comment.
Rooting is very easy. I simply cut that little branch, dipped it in honey (not entire branch, only part with the cut) and then I planted it in moist soil. Honey has natural growth hormones that can help rooting. As for the soil, it is best that it retains some water so that cutting doesn`t dry + always keep soil moist. Place cuttings in shade because sun would burn them. When you keep them in shade it is optimal situation for cutting to slowly produce food and develop roots. Direct sun would drain to much energy through the leafs.
Roots should form in 2-3 weeks. :)
In April I will make post on my private blog how to do that. (google: in between and undefined)
Last year my success was around 85% so this year I`ll planting around 180 lavenders into the field. That is a good rooting percentage since I found my lavender two days after it was cut down. I never tried rooting thick branches so I am not sure how that would work.