I'd seal the baggie, but open it for a while each day to let air into the bag. Sounds about right, but the only thing I would suggest is to use a seed starting mix instead of potting soil. Potting soil is too dense for good seed starting, but having said that, I have started plenty of seeds in potting soil. Just don't keep it too moist or you may have some damping off. Once you see green, open the bag and keep it open, but mist daily with a spray bottle. Once the plants have two sets of true leaves, start hardening off by moving outside giving increasing amounts of sun each day (e.g., 2 hours, 4 hours, 6 hours, etc.). Once hardened off, transplant in the garden.

this stuff is more of a mix than a soil mostly organics not really any dirt I think it will work

I'd leave it cracked.
I always germinate in a zip-lock with a moist paper towel.
Then I transfer the seeds to a mix of 1 part bark, 1 part pumice, and 1 part perlite.
I drape some plastic wrap over the top, put the flats under lights, and completely
remove the plastic when the first green seedlings sprout.

My method is quite "dry," you might say, and entirely free of peat-moss or potting soil.
My germination rate is nearly 100%, and I've never had a plant damp off.

Josh