Take a dinner plate, fill it with soapy water and lie the unplugged iron flat plate down in it. Let it soak a while, then with your finger, kitchen roll or a sponge (not anything abrasive) wipe off the burnt residue.
I used to get a product shaped like a crayon, which you rubbed onto the hot plate,then polished off , can't remember the name of it, but you could try Googling it ( my motto - if in doubt Google it). Good luck