Watch this science video tutorial from Nurd Rage on how to make iodine from sulfuric acid and alkali metal iodide. This is the best way to make elemental iodine from sulfuric acid and sodium or potassium iodide.
Warning: Reaction must be done outside or in a fumehood due to production of toxic gases including hydrogen sulfide.
First, get your sodium or potassium iodide and completely dissolve it in a minimum of water. Then place it into an ice bath to keep it cold.
Second, add concentrated sulfuric acid directly to the solution and constantly stir. As you add it will produce iodine.
At first the mixture will become pasty and eventually liquefy.
Once it's liquefied, pour it directly into cold water (the ice bath itself is useful for this) and stir it up. Then let it settle for ten minutes or so.
The iodine will settle to the bottom so pour off the water and acid.
The iodine will still be contaminated with acid so you'll need to add water and pour off several times to get out most of the acid.
Filter it and now you have a solid plug of iodine.
It's usable as-is for most purposes but you can purify by placing it in a beaker with a ice-filled round bottom flask on top. Then heat the beaker. The iodine will vaporize and deposit on the flask.
A cool reaction is to take some iodine and pour magnesium powder over it. It will react to form a purple cloud of iodine vapor. If it doesn't react quickly enough, just add a drop of water.
Brought to you by one of WonderHowTo's favorite scientists NurdRage.