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The best time to start seeds is 4 to 6 weeks before the last frost date. It can vary depending on the seeds. For example, are they all flower seeds or are some vegetables? Cool season vegetables can be started sooner than warm season vegetables, etc. We have lots of instructional articles on this website to help first timers like you so be sure to search our database for growing information. The propagator is like a mini greenhouse. Seeds are watered, then covered with plastic. Once the seedlings emerge, the plastic cover is removed.

Here are some to get you started:
https://www.gardeningknowhow.com/garden-how-to/propagation/seeds/when-to-start-seeds.htm

https://www.gardeningknowhow.com/garden-how-to/propagation/seeds/fungus-control-in-seed-trays.htm

https://www.gardeningknowhow.com/garden-how-to/propagation/seeds/leggy-seedling.htm

Each seed packet will contain different information for when to start each variety of seed. Generally, most things can be started indoors 4 to 6 weeks before the last frost date in your area. Some plants have other specific starting information. It is always best to consult your seed packet for information before planting. This article will give you more information:

https://www.gardeningknowhow.com/garden-how-to/propagation/seeds/when-to-start-seeds.htm

That could be a great way to start seedlings! I have done something similar with a rack in my home on many occasions. This can be useful for the temperatures, and weather won't cooperate.

You can start in the terminal pot, but the problem arises if you are not familiar with how to water such a young seedling. Since the plant is very young, it will not need the entire container to be wet. It won't dry out often enough to give proper root formation fast enough. If you can only water what is necessary, and it is dry within 2 or 3 days of watering, then it won't be a problem. It is just much easier to get a good watering amount pinned down with a "just fit" pot, but it is not impossible to start them in the terminal pot.

Do they germinate? If not, I would try putting the baggie near or on a windowsill. The fridge may be too cold. If your method is working for you, that's great. You didn't mention what kinds of seeds you're germinating. Here's more information:

https://www.gardeningknowhow.com/garden-how-to/propagation/seeds/seed-germination-terms.htm

https://www.gardeningknowhow.com/garden-how-to/propagation/seeds/when-to-start-seeds.htm

Generally, you want to wait until those pods are starting to crack open a little on their own. This will let you know that the seeds are ready for germination, and mature.

You can still try and germinate those seeds, but you will want to let them dry out, first.