It’s still a little too cold in the Northeastern US to plant much outside. I have peas and kale, who love colder spring weather, started outdoors. In the meantime, I’ve run out of front window space! Seedlings have taken over!
A plastic egg carton makes a very good seed-starting greenhouse. Most plastic egg cartons have three sections- one that goes under the eggs, one that goes over the eggs, and the lid. Cut off the one that goes over the eggs with scissors and then poke holes at the bottom of it with a knife. That piece will then fit right into the section that goes over the eggs, so that the one with holes in it can hold dirt and drain water, and the one underneath it can capture the water. Until the seeds sprout and get too tall, you can use the lid as the top of your greenhouse.
Editing to add some pics from my cellphone to make this simple process more clear: