If you can still find iodine crystals, this is how I remember doing it. I borrowed this from another site. I think if first learned about this in a wilderness first aid class.

"Using Iodine crystals Water Purification
equipment
1. A 1 oz. glass jar with a bakelite cap (iodine is destroyed by contact plastic or metal).
2. A crystal of Iodine 0.1 gram minimum but more just insures a saturated solution.

1. Add Iodine crystals to 1 fluid oz. (1 tablespoon) of water (at 70 to100oF)
2. Close and shake for several minutes until you have a saturated solution with as much solid has dissolved as will dissolve.
Treating Water
3. To treat 1 liter or quart of water, carefully pour off 10ml (2 teaspoons) of the saturated iodine solution.
4. Caution: iodine crystals are VERY TOXIC! Do not tip the bottle over too far and spill crystals into the drinking water.
5. Let stand at least 15 minutes at room temperature or higher, making sure all of the interior surface including lid get treated.
6. Note: Iodine is not completely effective in destroying Giardia in cold water. The efficacy of iodine depends on contact time; 60 minutes at 60F, 30 minutes at 80F and 25 minutes at 115F for 100% destruction of Giardia cysts. If you have beavers, assume Giardia in the water.
7. Adding a pinch of Vitamin C (ascorbic acid) after the treatment helps eliminate the iodine taste of the water.
8. Iodine treatment appears safe for 3-6 months of use, but questions remain about long-term usage. It should not be used by persons with allergy to iodine, persons with active thyroid disease, or pregnant women."