You can construct a small DIY greenhouse for propagating seedlings when you pair a plastic bag with a planter. First plant seeds in a 5-gallon bucket, or the container of your choice. Put 3 or 4 sticks into the soil and loosely place a plastic grocery bag over the top. Once you've added your bag, keep it in place by sealing it to the container with duct tape. Once the sprouts are large enough, remove the bag and transplant the seedlings to an outdoor garden or a sunny windowsill.