Alright, here goes
Before I tell you the steps, here are a few things to keep in mind.
1) The firing of an ice-block almost functions like a DJC, in that it changes the momentum of your jump. However, what it does is it almost functions like a jump of its own and gives you a set amount of height, then drops you. In short, if you fire it while you're still rising, it changes the height of your jump.
2) If you land before the ice-block comes out fully, it materializes but doesn't fire. It won't hit anybody, but you DO get hitboxes from your hammer swing.
3) To platform cancel, you need horizontal momentum; if that momentum carries you to the edge of the platform, it will stop your lag by replacing it with a new animation. If you go backwards, you just fall off the platform and so there is no more landing lag. If you go forwards, it will be replaced by the "Oh crap I'm falling" ledge balancing thing. We need a name for that, btw. I vote for OSACA, which stands for "'Oh ****, a cliff' animation."
4) When you fire iceblocks early in your jump, you retain a fair amount of horizontal momentum.
Alright, glad we got that covered.
So, to platform cancel an iceblock, you need to position yourself beneath the platform. Next, you do a dashing full jump. Somewhere in there, depending on the height of the platform, you hit B to fire your ice-block. If you do it right, you will go forward firing the ice-block then land on the edge of the platform and have no lag. If you go backwards you will slide off, if you go forwards you will experience the OSACA animation and you will have to hit forward to run off the platform.
You can also do this by sitting on a platform and wavedashing into an ice-block. This is, IMO, a lot harder to do because it requires you to time the wavedash perfectly and the iceblock perfectly as well as space yourself on the platform exactly right. Laaaame. Note that you don't experience OSACA doing a wavedash ice-block.
I hope that helps.