Mark the CG location you are happy with as a land based plane i.e. where the plane hangs level.

Now add the floats with the step directly below the marked CG. Slightly behind is okay too.

Now hang the plane by the marked CG location and add weight to the front or back of the floats to level the plane. Adding the weight to the floats rather than to the plane allows you to switch back and forth between land and water (including snow don't forget) with out adding and removing weight. Of course, if you're really lucky, you won't need to add any.

For other important considerations check out
http://www.flyinglindy.homestead.com/skisandfloats.html
I followed Chuck Cunningham's articles years ago and my first attempt, and everyone since then, has worked beautifully.