Roy outlined the steps in collecting seed from soft fruits. The process, which involves fermentation, simulates the conditions of a rotting fruit that's dropped to the ground or has been eaten by an animal and passed through its digestive system. Seed saved from hybrid fruit such as that bought at the greengrocer may not come true, that is produce identical fruit. This is easier to arrange in your own vegetable garden. Shop-bought produce, although it can be used for seed, may have been harvested before the seed it contains becomes fully mature.
Immature seed is less likely to germinate.
Green capsicums for instance are immature. When they are ripe they are fully red and may be a little soft. Remove the seed from inside the fruit. This can be done by spooning or squeezing out the pulp from the seed cavities and placing it in a small bowl. Add a little water and allow the pulp mixture to ferment in a warm spot for about five days.