The basic of the model is the knight champion, the musician's horn arm, 2 plastic skulls from the chaos warrior box, 2 x drum plus drumstick arm as well as a weapon-carrying left arm from a box of plastic Chaos Marauders. To fill out in the gaps between the rider and the drums I added a bits from the Kroot plastic sprue, but other ramdom bits of appropriate size should work. The greenstuff work involved: A little armour on the left arm, the robe on the drumsticks, a pouch, and some filling on the drums (where the Marauder arm holding the drum was cut away).
In all, I think that the conversion is well worth doing (and if you have got the Marauders box you should have 3 sets of spare drum gear!) and adds a lot of character, whereas the musician that comes in the box is hardly noticeable.