To grow tempeh, you need a tempeh starter, which is basically a culture of Rhizopus spores, the reproductive agents of the mushroom. You can buy them ready to use in several shops, we sell some in our e-shop.
Make your own
You can also overripen one of your tempeh so that it matures to sporulation (that is fruiting in mushrooms) and harvest the spores to use as a starter. Sporulation occurs after mycelial growth. In tempeh sporulation is marked by darkening color that corresponds to the abundance or microscopic black fungi. Higher oxygenation and drier conditions favour sporulation, which is why sporulation will almost always develop on tempeh near the holes in your mould. To maximise spore production, you can expose as much surface area as possible by slicing your tempeh block and leaving it in the incubator, outside the mould. When it is fully sporulated and dried, you can simply grind it and use it as a starter. Note that each generation of starter has the possibility of contamination by bacteria and weakening of the mold. Maintaining a pure culture requires a scrupulous and methodical effort. Commercial starters are manufactured in laboratories under sterile conditions and each batch is tested for low levels of unwanted bacteria.
Use your mycelium
It is also possible to make tempeh without starter but from a piece of fresh live tempeh (non pastheurised!). Mince it finely and mix about 10% of it with your cooked and dry beans. The mycelium will then continue to grow without the help of spores. Note that the mycelium will be weaker than with the use of starter and will need a longer incubation time with more risk of contamination.