Someone asked for this, so I thought I would start one and others can chime in. If the mods like this, they can sticky-fy it. All of this has been written in other threads here, but I think not all gathered up in one spot.
The basics of what you need for starting seeds is good light fluffy "soil" (usually soil-less mix), light, heat, consistent moisture, containers.
: The basic Miracle Gro potting soil works fine. Or you can make your own mix with peat moss or preferably coconut coir (a non-mined, more renewable product), perlite and/or vermiculite, compost. They sell seed starting mix which is very fine textured, but has no nutrients. If you use this, you will need to start fertilizing once the seedlings have true leaves.
: Does not need to be fancy grow lights. Most often people just use ordinary fluorescent tubes in cheap shop light fixtures. This would probably not be good enough light for growing fruiting plants to maturity and fruiting, but is great for starting seeds. But the lights have to be right down close to the plants, like just 2-3" above, hung on chains so that they can be raised as the plants grow. They should be on 16 hrs a day (some people even do 18, but 24 hrs is NOT better, they do need the rest period). Note that some very fine seeds like petunias and impatiens need light to germinate. Do not cover these seeds at all, just very lightly press them down so they are in contact with the soil.
: Most (but not all) of the garden seeds that people would typically be growing germinate much better (faster, more reliably, better germination rate) with soil temperatures above 72. There is a sticky in this section with a chart of germination temps. The seed packet will usually tell you what temp the particular plant needs for germination. For plants that need warm soil, you need to have a way to provide that. They sell special plant heat mats, but regular pharmacy heating pads sold for people work just fine as long as they don't have automatic timed cut off. Or some people have had good results keeping them on top of refrigerator (but that is usually a dark spot, so you would need to watch carefully and move them to the lights as soon as sprouted) or using incandescent light bulb under the shelf they are on to provide heat, etc.
: This is probably the trickiest part, kills most seedlings. Baby seedlings have very little root system, so cannot be allowed to dry out or they die. However they are also very sensitive to being too wet. There is a fungal condition called damping off that young seedlings are prone to in conditions of low air circulation and too much moisture/ humidity. The stem starts kind of rotting just above soil level, gets a little bit pinched in and the seedling keels over, ends up lying flat on the soil. Once that happens, it is kaputt, all you can do is get rid of it. So you need to provide consistent barely damp, preferably with good air circulation. Easiest way to do that is by "bottom watering": having all your cells/ pots/ containers in trays. Then you just pour a little water into the bottom of the tray, just until it just touches the bottom of the pots, so the soil in the pot can wick it up. Probably add a little bit of water each day, but NOT if there is any water left from the previous day.
: NOT peat pots. They are a menace. Hold too much moisture and stay too wet and then suddenly dry out and suck all the water away from the plant. And if you are bottom watering, they will get all moldy on the bottom. Use plastic. Can be the little cells and pots made for the purpose or it can be plastic yoghurt cups, drink cups, or whatever is handy as long as you put plenty of drainage holes in them. If you are using heat mats, it is typical to start plants in the little grow cells, so that you can crowd a lot of seeds onto one mat. If you do that, once they are well sprouted and have their first pair of true leaves they will need to be transplanted out into little pots.
AVOID the little seed starting kits sold with domes and peat pots, they are seedling killers! I don't use a dome at all, to easy to damp off your seedlings. If you do use a dome, remove it as soon as the seedlings are sprouted.
The main other thing you need to be aware of for seed starting is TIMING. If you start seeds too early, they will out grow your space before it is warm enough outside to put them out and you will end up with long spindly plants. If you start them too late, you will end up putting them out in the heat of summer. You need to be aware of the difference between cold weather crops, which are frost tolerant, like cool weather and tend to fizzle as soon as it gets hot and warm weather crops which die in frost and like hot sunny weather. Cool weather crops include peas, brassicas [broccoli, cabbage, cauliflower, etc], root crops like potatoes and carrots, and green leafies [lettuce, spinach, chard, kale, etc]. Warm season crops is pretty much everything else. Corn and beans are earliest of warm season, then tomatoes and peppers, and the cucurbits[squash, melons, cucumbers, etc], are the latest, need the soil really nice and toasty warm. So you want to have your seedlings ready to go out at the right time for the season they like.
At this site it will tell you when your average last frost date is:
https://www.almanac.com/content/frost-ch ... ted-states
The seed packet will tell you how many weeks ahead of that date to plant your seed. That will at least give you a good starting point until you have more experience with your own garden.
That's all the basics. With more experience, you will learn nuances, like grouping things together in trays that have similar requirements (especially for moisture vs drought tolerance), but this is enough to get you going.
Anyone else can chime in, with their experience/ tips.
(Bottom shelf right side, you can just see a couple blue lines with cords coming out from them. These are the heating mats. I have everything coming down to power strips so that I can turn all those lights on and off with one button. The mats are on a different strip so they stay on 24/7)