There are two ways to go. Either with round filters by buying the largest size you need, and getting the step rings, or by buying into one of the square filter systems from Cokin or Lee, etc.
In either event, you will likely not be using the lens hood that came with the lens, because a too large filter will end up preventing you from mounting it.
If you plan on shooting landscapes, you might end up with a square system, anyway, so you can use graduated ND filters to best effect.
I bought into the Cokin system, but for circular polarizers, I have filters for two of my lens thread sizes. The Cokin is nice, and necessary for some things, but I'm leaning toward the individual size round filters, as you can keep the lens hood, and they are much quicker to mount & dismount. And, if you are paying Canon "L" lens prices, what's a hundred bucks or so for a good filter?