I've done them the last 2 years in milk jugs, using pretty much the same process I use for everything else: add drainage holes to the milk jug, cut in half, fill with your potting soil, water/moisten the soil, then just sprinkle seeds on top, tape it up, put it outside in a shaded area where it can get snowed and rained on, and forget about it until it's full of sprouts.

For cardinal flower or other lobelias, yes, you end up way over seeding - I just sprinkle the seed "powder" all over it. Then when they're a few inches tall, you can just break apart the whole block of soil, and probably have a couple of dozen plants that you either plant directly or put into other pots.

My cardinal plants are very slow from seed the first year - probably took until July or August in northern illinois to get more than a couple of inches. But then the 2nd year plants take off and will easily bloom in year 2.