Don't melt it directly in a pan on the heat, that is a surefire way to wreck the chocolate.
I have two suggestions - one would be to melt in a double boiler (glass bowl over a pan of simmering water), the second would be to put the chocolate on to a lined swiss roll pan in a very low oven, possibly with the door open a little, depending on how low your oven goes. Keep a close eye on it, and once it has started to melt, help it along with a palette knife until it spreads across the whole pan. Leave it to cool a little, and then score lines (as you would see on a bar of chocolate), and then leave to harden fully, and then break into pieces.