While harvesting the seeds from a banana is simplicity itself, finding a banana with seeds may prove to be more difficult. Commercial bananas grown for human consumption have been bred not to have seeds. In order to find bananas with seeds you will either have to buy plantains, an early form of modern bananas, or you will need to obtain bananas that have cross-pollinated with wild bananas or ask your grocer to special-order bananas with seeds.
Purchase plantains or "wild" bananas that have not had the seeds bred out of them. Finding "wild" bananas is not always easy, but you may be able to find some at stores that specialize in "natural" fruits or your grocer may be able to special-order them for you.
Wait until the bananas are completely yellow with small brown spots just starting to appear so you know the bananas are ripe and the seeds within are mature. Peel the banana or the plantain and discard the peel.
Place the banana or the plantain into a bowl and use the back of a spoon to carefully mash the pulp, being careful not to smash any seeds. Seeds will be brown to black and may be a variety of shapes, though predominately roundish or slightly oblong.
Add water to your bowl of mashed-up banana and use your fingers to separate the pulp from the seeds. Pick the seeds out and place them on a paper towel. Discard the mashed-up pulp.
Put the seeds in a fine-meshed strainer with a mesh too fine for the seeds to wash through. Wash the seeds under cold running water until they are clean. Place the seeds on a paper towel to dry.