We used to grow and sell B. colvilei 'Kewensis' when I had my nursery.
I used to take semi-ripe side-shoots with a small heel, wound them
(take a fine slice about 1cm. long from the side of the stem
immediately above the heel) and insert in trays of 60/40 perlite and
moss peat. They were drenched with fungicide several times at weekly
intervals and kept in an part-open sided polythene tent on the prop
bench without basal heat, watering only when the compost started to dry
out. Winter temperatures in that house were maintained a few degrees
above freezing on cold days.

Apart from picking over dead or yellow leaves and ensuring they didn't
become bone dry, they were left to their own devices until the
following spring. By then, most of the cuttings had calloused well,
but with little if any actual root. They were then shifted to a
slightly heated bench to encourage root action. Only when several
leaves had been produced were they potted on, but after that they
usually grew away quite well. 'Kewensis' tends to flower earlier than
the species and is a somewhat richer colour, so a plant in a 20L pot
with one of two flower spikes would be ready for sale a year later.