Tie the bag with a knot or use a strong rubber band. Then put it in another bag and seal it. Leave a little room in the bag for the air to expand when it is in the air. You don't want a bag to explode. You can add water and skin conditioners if you so desire, they certainly won't hurt anything.
Ammonia absorbents are also good and help in keeping the fish from stressing out.
Other medications such as Acriflavine or Methylene blue are options, I have never used them myself. Tranquilizers like Hypno or Trance are suggested for larger fish, it slows the metabolism down, it usually lasts for 48 hours or when the fish is put into fresh water. Use your good judgement when adding anything to the fishes water. After you have properly labeled your bags, you can put them in your shipping box. You can buy them from the stores, or make your own.