Growing Cotyledon
I must admit that I feel a lot less confident about growing Cotyledon in a cold west European climate. Nevertheless I will give it a go as there are certein aspects I am sure of.
Cotyledon pendens is sold online
Propagation
cuttings
Unfortunately Cotyledons cannot be propagated from a single leaf like Adromischus. If that would be the case, one would find a lot more of them. I takes a stem cutting.
Just cut a 10cm piece of a well established stem with a sterile knife and let the cutting dry for at least a week in a shaded dry spot. Then pot it up in a normal medium and do not water. Wait several weeks before giving some careful water. Watch the leaves at the tip of the plant: if they start to grow erect, your cutting has taken and you may continue to water carefully. Eventually move the plant to a more bright place.
That being said, stem cuttings grow fairly easy.
On the other hand, be careful not to take too many cuttings! I killed a plant like that, by 'pruning' it like a hedge in the idea that I would produce 30 cuttings in one go... disaster: all cuttings and the mother plant died....
seeds
If you can get them, Cotyledons can be sown. They grow fast and within three years you have a flowering plant.
Growth conditions
Light
Cotyledons generally should be grown in very bright light, some in 100% full sun and beyond: some forms of C. orbiculata and C. papillaris grow in true desert conditions which can fry an egg. In my greenhouse (in Belgium) I try to give them the brighest spots, for instance in a hanging basket close to the roof. They will certainly grow well in more shaded places, but to keep them compact and tight, bright is best.
Of course, be a bit more easy on young plants.
Soil
Cotyledons are happy in a normal 'cactus' mixture. They have fine roots and therefore seem to like a measure of coarse sand and gritt in the mix (builders sand).
Water
During the growing season I water on average once every three weeks. In very hot weather an occasional spray with the garden hose may be given in the evening. During winter (november - march) I do not water at all, I do not want them to grow.
Air
Like any succulents, Cotyledon enjoy fresh air. So as soon as temps outside are above 20C open windows and doors in your greenhouse.
If you have a place outside which is shielded from the rain, for instance a terrace, by all means, put your plant outside. It will love it.
Temperature
Cotyledon grow well from 20°C onwards. Most withstand minimum temperatures of 5°C or even less.
However be careful with C. barbeyi : one tends to put it outside in summer as it forms a big plant. It is much more sensitive to coldsnaps than for instance Crassula ovata or Crassula arborescens. Put it inside before any frost happens.