make sure that your sheet of lego will roll around your wrist as a cuff, and the plastic shrinks when you heat it so check first! mine was a shee so i had to cut it up, i snapped it then cut it so it had rounded edges etc.
i used a nail file, but sand it back a bit so that it's smooth.
dont worry if where youve sanded/cut it is white, when you boil it this will go away. (bad pic but ...)
boil the water and put in the lego sheet. it takes a while for it to soften up, just watch it and check it so that it doesnt melt, it took around 3 minutes for mine, but i had a bit of trouble.
once you have bent it into the right shape for your wrist, put it in the cold water.
get your lego man ready with his bits and pieces, mines a robin hood/merry men one :D glue gun him all together so you dont lose any pieces when youre wearing your cuff. i filled his torso with glue too.
sadly, you cant interchange pieces onto your cuff, as the plastic shrinks in the hot water. so you have to glue your little guy right on there. i make mine lie down a bit, just to keep him from hooking onto stuff :D so there you go :D enjoy!