As understand it, propagation of cacti is normally achieved via cuttings. But if you're concerned about seed saving then, to me, the question is how to induce flowers. Then worry about fertilisation, fruiting and seed collection.
Check you've optimal growing conditions so it's not afaid to flower. Optimal conditions for Schlumbergera cultivars, based on Christmas Cacti : The genus Schlumbergera and its hybrids (ISBN 9780951723463) by AJS McMillan and JF Horobin and mentioned on Wikipedia:
- Plant is grown in free-draining, humus-rich, somewhat acid soil
- Plant is kept just moist throughout the year to avoid underwatering or overwatering
- Plant does not recieve much light (Truncata is less tolerant of excess than other Schlumbergera) [I am not sure how McMillan and Horobin quantified the amount of light]
Check how much light your Schlumbergera is receiving:
- Very low light levels will prevent flowering.
- Continuous darkness for at least 12 hours is necessary to induce bud formation. (8 days with 16 hours of darkness at 16 °C (61 °F) has been shown to cause buds to form).
- Temperatures lower than 16 °C (61 °F) slow the process of bud formation. Guide-to-houseplants.com advises:
To set flower buds, the plant needs cool 60-65°F/16-18°C days and 45-55°F/7-13°C nights. Once buds set, 70-75°F/21-24°C days and 60-70°F/16-21°C nights.
- Withholding water from the plant has been shown not to speed bud formation.
Note also: From Cacti: biology and uses (ISBN 9780520231573) by Park S. Nobel, the following points are made (condensed by me):
- "Both flower number and fruit number dramatically increase in response to both mineral and organic fertilizers, water, and pruning". Guide-to-houseplants.com advises:
Feed every 2 weeks with a balanced liquid fertilizer diluted by half. After blooms have dropped, stop fertilizing for a month.
- "Low temperatures in the winter influence flower bud burst for some cultivated cacti, indicating that chilling is involved" quoting "Nerd and Mizrahi 1997", but since buds appear all year round in the Peruvian highlands, no one is really sure what other physiological factors are involved.
Hat tip to @EricNitardy for the advice that Schlumbergera typically require cross-pollination to produce fruit (per CentralArizonaCactus.org advice on "holiday" cultivars of Schlumbergera and confirmed in the AJS McMillan and JF Horobin resource mentioned above), although most other cacti can self-pollinate. So you need a second Schlumbergera truncata specimen that's not a clone (can be sure if it has different coloured flowers).
Then you can either leave it up to the birds and bees to cart pollen between the flowers of the two plants or manually fertilise by the following steps (which is the same for many many plants if they're propagating sexually rather than asexually or by cloning):
Identify the stigma of a flower where you'd like to see fruit. It should be deep red in colour and protrude from the flower (per pictures and advice from Mr Brown Thumb).
Identify the pollen on a flower of the other cactus plant. It is a yellow clump hanging off of the end of the filament (white tube-like part of the flower's stamen; again see Mr Brown Thumb images).
Take the flower containing pollen and coat the stigma of the first flower with pollen.
Hopefully fruit will start to grow at the base of the flower bloom (see pictures on Random plant event: Schlumbergera fruits blog post).