Perhaps one of the most important stage is pollination.You must take pollen from the stamen of one of the parents and put it on the pistil of the other. Cactus flowers generally live only one or two days, nevertheless you can refrigerate pollen and use it after many days. Not all crosses will give you seeds : it's easy to cross cacti of the same genus - e.g. Lobivia x Lobivia -, it's easy enought
to cross cacti of
similar (in a evolutive meaning) genera - e.g. most of crosses
you can do between Lobivias, Trichocereus and Echinopsis will
produce seeds - while you will hardly be able to cross e.g.
Echinopsis x Echinocereus. Nevertheless (even if rarely) somebody
is able to produce hybrids (and generally they are the most
interesting) between very far genera. In literature (e.g.
Innes,Glass-The Illustrated Encyclopedia of cacty -, Cactus&co. journal vol.II n.4 December 1998,......)
you can find photos of Epiphillum x Echinopsis , Leuchtenbergia x
Ferocactus , Hildewintera x Echinopsis ,Hildewintera x Lobivia,
Heliocereus x Discocactus , I have tried succesfully to cross
Hylocereus hundatus x Trichocereus "Blüte wie Wüstenglut"
(in the photo you can see a 2 years old plant), while I have always obtained barren seeds from crossing of my Epi Ackermanii x Echinopsis or x Tricho.