Here is how I prep a Ziplock Freezer bag for sowing
What I do is cut the bottom corners off the ziplock bag and make wide fringes across the bottom crease for extra drainage. Fill bag about half or a little more with seed starting mix (it will compact with freeze/thaw cycles so adding a bit extra won't hurt, water the mix and sprinkle seeds on mix and press into place and then sprinkle a bit of mix on top to cover the seeds if need be.
And when you're ready to take it outside zip the bag almost shut to still allow air flow. Once it starts getting warmer out you can zip the bag more and more open until you can just fold the bag open like a cuff. Perennial and hardy annulas you can do now.
Hope this helps!
