Cross-pollination is simply the process of taking pollen from the flowers on a plant and using it to pollinate another plant's flowers. This is the most common means of pollination in nature. and is a natural method of preventing inbreeding. Botanists commonly use this method to single out certain desirable traits in flowers, vegetables, trees, fruits and herbs, forming hybrid varieties often found in commercial nurseries. It requires the use of external forces, whether it be wind, insects, animals or, in this case, humans.
Choose two varieties of flowers that you wish to cross-pollinate. Work with two separate plants, preferably within the same species. For instance, if you wish to cross within petunias, try using two separate colors from different plants, like white and red to produce a pink-flowered petunia.
Use a soft-bristled paintbrush to gently swish the inside of the blossom of one flower, then walk immediately to the other flower, and do the same thing to it. You will have now transferred the pollen from one flower to the second. Do this with several flowers to make sure you have a successful cross-pollination.
Wrap the blossom of the receiving flower so that the whole flower is covered with a plastic baggie, and attach it with a piece of tape around the stem just below the flower. This will protect the flower from receiving pollen from another flower by a passing bee or ant, or even the wind. Inflate the bag a little so that you do not constrict the blossom too much by the bag.
Remove the bag when you see the flower has faded and wilted and the ovule has swollen with seeds. Different flowers have different sized seeds, but if you compare it with other flowers on the plant, it should have a larger base. Be careful not to break the seed head from the branch, as the seeds need to ripen before you pick them.
Collect the seeds when the seadhead has dried and turned a light brown by clipping it from the flower and placing in a dry container. Open the seed head by squeezing it between your thumb and forefinger and inspect the seeds. They should be hard and plump, and look like the seeds of the species you are working with.
Plant these seeds to see if your cross-pollination attempt has produced the results you were trying to achieve. The more seeds you plant, the more chances you have of reaching a desired effect. Keep good records using pictures if possible to keep track of your experiments. A simple notebook with your descriptions and a picture taped to the page will work as well as a document on your computer.