I've done this before by the method suggested by the pp--blanch (very quickly in boiling water, then into an ice water bath). I froze in small amounts in baggies. You can use where you would use frozen spinach but IMO they are not great on their own--better as a part of something else (lasagna, quiche, or what have you). When using them, try to squeeze a lot of the water out otherwise it might wreck your recipe.
We had tons of kale in the garden last year, and I froze a bunch like this. Checked the freezer, there are still a few baggies almost a year later! I guess that probably shows you that they are not at their yummy-est when frozen!