Why spend so much money every year in buying new plants and plant seeds when you can collect everything you need for next year in your own backyard. And since most of these plants have cross-pollination, so with free seeds you get surprisingly new varieties of flowers.

preparations you must make well before collecting seeds:

• If you plan to collect seeds then obviously you should leave some flowers on the plant. So don't dead-head every single peace because you CAN'T collect seeds from dead flowers.
• Buy some paper envelops to store seeds. I make my own with old newspapers. Paper envelops are best because they allow moisture to escape and lets seeds have some air.
• Keep a pen or pencil handy to label the envelops as you collect seeds so that you don't mix your white cosmos with pink ones.
• Use small stick or scissors to take out seeds.
  
• Remember you won't get the same plant from these seeds if open pollination has occurred. So be readily to be surprised.
• Let the seed pods mature and dry out on the plant before collecting. The color will change from green to brown. Seeds collected from green seed pods won't germinate.
• If you are worried about loosing your seed then cut the flower and put it in a paper bag and let it dry.
  

Two methods of collecting the seeds:
Bagging:This method is particularly useful for seeds that are really small. Or you don't like seeds sticking in your nail. Just cut the whole seed pod and put into a paper bag. Crush the seed pod gently and take out the husk and remains of seed pod out. Seal the bag with some glue.
Direct collecting: This is the most common method of collecting the seeds which are big enough to handle with hands. Just separate the seeds from seed pods on a big piece of paper or plate (preferable white in color if the seeds are relatively small). Now transfer the seeds to the bags and seal the bag.

These are just few tips and methods of collecting seeds. With time and patience you can achieve some good results.