Pour vinegar (or lemon juice) in it, turned off, in horizontal position (flat) in an empty washing up bowl with the iron set to steam. It should dribble out of the steam holes and de clog them, leave it for a few hours. Worked for me.
I don't think you need much, as it should sit in the stuff as well so will unclog that way. Get a steam generator with a built in descaler that doesn't need replacing, it just collects the calcium in a reservoir that you empty, excellent invention!