Start with a quick 600 grit (metal sand paper not wood sand paper) and finsh with 1000 grit.. You wouldn't want to start with a 180 or 320 grit, you'll end up sanding the edges off!!! You could start with a quick 400 grit, but the 600 just takes a lil longer but its more user friendly.. Look for wet n dry sand paper(almost all metal sandpaper is) and keep a water spray bottle or wet rag next to you, and wet the spot your sanding just a lil bit, it sands easyer wet and you won't go thru as much paper. And like someone said use a block to sand, wood block any thing the size of an I-pone, wrap the sand paper around the block, the block keeps a straight n true edge, as long as you keep moving and not stay in one spot for to long.. Let the sand paper do the work and get a nice smooth edge, don't use too much pressure. And once you start getting an smooth edge, move to the 1000 grit and finsh it off.. You could go up to 1500/2000 grit but 1000s reall fine/smooth. If you have any other probs PM me.
A good way to sand is by making X's.. Like a 45%, start from bottom left and end up at top right... Vice versa,, start from bottom right top left. Like X's to ROUND the edge and when your done then you could run the sand paper down the ski from tip to tall, to clean it up..