The way I clean the bottom of my steam iron is I take some baking soda and water and make a paste out of it.
Mix 2 tablespoons of baking soda with 1 tablespoon of water to make a paste.
I use a spatula to apply the baking soda paste to the steam iron and then use a damp cloth to wipe the steam iron clean and also cleaning the baking soda off and then use a q tip and dip it in some water and use the q tip of cotton swab to clean out the holes.
Then pour clean water into your steam iron and then use the steam to push the deposits out of the holes.
You may have to wipe the bottom of the iron a few times and scrub it with the cloth and baking soda to get a really dirty steam iron clean.