Some of the easiest tried-and-true seeds to begin with are leafy greens like lettuce and kale, or hardy veggies like broccoli and cabbage. Cold hardy perennial flowers do great as well.
Choose a container that's clear (or only slightly opaque) to allow sunlight to penetrate. Make sure there’s enough room for root growth (can fit at least 3-4 inches of soil). It will need holes for drainage and ventilation, but you can also cut or drill these yourself.
Fill your chosen container with at least 3-4 inches of potting soil. Next, moisten the soil thoroughly, but not soaking wet. Now you can sprinkle your seeds! You can plant them densely and thin them out later.
For Ziplocs, close the top zipper most of the way, leaving a small section open for airflow. You can use clothespins to keep it open if needed. Next, label your containers!
Set your containers somewhere they’ll get plenty of winter sun and exposure to the elements. Rain, snow, and wind are good for them, and this exposure to outdoor conditions will result in hardy, robust seedlings!
If you have plenty of rain and snow, you can ignore your containers until it’s time to plant in spring. If your winter climate is dry, you should check and water them as needed to keep them moist.
Once all danger of frost has passed, you can transplant your seedlings directly into the garden. Gently separate their roots, or plant them in one big hunk, depending on how many you planted and their preferred spacing.