When starting out with growing from seed it's easy to go way get caught up and buy everything on a shelf to help you grow from seed. One of the things that I always want to buy are those plastic mini-greenhouses for seed starting. The thing that keeps me from buying them is the small size and usually the price.
I guess I'm too cheap to spend the money on them, so like many people before me I decide to make my own. The one in the photos attached is made from an ordinary soda bottle I cut in-half, poked drainage holes, filled with soil and added seeds to it. The top I just slid inside the bottom half to close the bottle back up. I know some people use tape to seal it but I don't find it necessary. If you cut a few vertical slits along the rim of the bottom half it will make it easier to slide the top half down.
What you end up with is basically a little seed starting greenhouse that works just as well as the ones you buy at the garden centers but it only cost you about a dollar. The money you save you can spend on more seeds, pots, soil etc.
I leave my tops off of my home made greenhouses, and I find that even with the top off enough humidity gets trapped while allowing air circulation. No need to constantly be venting like with the commercially available ones.
I've added a picture of what the seedlings look like inside. The seedling you see are mixed Scabiosa seeds I traded for.