Indoor seed starting can be intimidating... but it really doesn't need to be. When I first started getting really serious about gardening I watched tons of YouTube videos about intricate, expensive seed starting systems. I've tried a lot of different seed starting products and a lot of them are really bad. There is truly nothing more discouraging than getting excited about all your new supplies and seeds to just be disappointed by leggy, moldy, or un-germinated seeds and seedlings. Through trial and lots of error I have created my own failproof seed starting system. Starting seeds can be quite like Goldilocks - not enough heat, too much water, not enough air flow, not enough light, and so on. But with this seed starting system, you can easily accommodate the ever fickle seedling.
Choosing a rack, cart or table
You'll need a good cart, table or rack to start your seedlings on. Choose your surface based on how many seedlings you want to start and what suits your growing space. For me, I had an unused corner of my home that fit a large rack. If you plan on using the common 10x20 inch seedlings trays, I suggest purchasing your rack with that in mind. My rack is 60 inches wide and 20 inches deep so I can easily fit 5 seedling trays per shelf.
choosing a heat source
The primary reason to start seeds indoors is to get a head start on the growing season while it is still quite cold outside. Spring and summer vegetable and herbs like tomatoes and peppers require 70 degree soil temperatures in order to germinate. There are couple of ways you can achieve warm enough soil temperatures to ensure your seeds germinate. The easiest, most reliable option is to use seedling heat mats. The seedling heat mats go directly under your 10x20 trays. You don't need to get too many seed mats either - once your seeds germinate and develop their first set of "true leaves" you can move the heat mat to another batch of seeds. True leaves are the first set of leaves that emerge after the initial "cotyledon leaves."
If you have a spot inside your home that gets enough direct sunlight throughout the day, you can also try to start your seeds here. You'll want to make sure you use a humidity dome on your seed trays to maintain warmth. Bottom line, you want to make sure you can create an environment that is consistently above 70 degrees to ensure your seeds germinate.
Trays and pots
You have a couple options when it comes to trays but I highly recommend 10 x 20 trays WITHOUT allocated seedlings compartments. Essentially, it's just a plastic tray. I prefer these trays to ones with allocated seedling compartments because they offer you more flexibility. You can move around seedlings that need more light, more ventilation, extra nutrients, etc.
When it comes to pots, you also have a lot of options. I've tried nearly every method - biodegradable pots, soil blocking, etc - you name it. 3" nursery pots just do the job with no fuss. You can fit 18 3" square nursery pots perfectly on a 10x20 tray.
I like these labels: they're waterproof and suit the labels that my label maker creates. Learn from my mistakes... label everything! Even if you ~think~ you'll remember what you planted, you won't.
To label my seedlings I include the plant type, specific variety, and date sown.
I never thought I could be so passionate about a watering can?? I picked this one up on a whim from my local hardware store. It's perfect. Durable, light weight, multiple handles, good water stream. It's a match made in heaven. After I fill my pots up with soil and sow my seeds, I water them enough to wet the soil through. To germinate seeds, you need to make sure the soil is consistently moist. Once the seeds germinate, begin to water lightly when the top 1/2 inch of soil gets dry.
To get things nice and steamy in your seed trays, I like to spritz the seeds often with some water. Using a spray bottle is particularly useful when using humidity domes. You can just spray the lids and pop them back on and they'll drip down on your seedlings.
After heat, ventilation is the next most critical element of seed starting. Leggy seedlings are the product of poor ventilation. Seedlings must be "hardened off" before transplanting them in ground. Essentially, you need to get your seedlings accustomed to what the real world is going to be like. If you start your seedlings near a window you can just pop open the window when it's reasonably temperate outside. You can also bring them outside as long as it isn't too windy or cold out. Lastly, you can supplement by using a fan to create ambient air flow.
I left this for the end of the post because grow lights are really expensive compared to everything else. I haven't found grow lights that I love enough to truly promote or endorse. What I prefer to do, if I have time, is bring my seedlings outside for the day if it is warm enough. In Southern California, it isn't uncommon for us to have many warm, sunny days during winter and early spring. On other days, I move my seedlings to a warm, sunny spot indoors. When that doesn't do the trick, I supplement with grow lights as needed.
What feels like the millionth time, again, you have options when it comes to your starter mix. I've tried coconut mixes, soil blocks, etc. Personally, I haven't found anything that makes me as happy as the Dr. Earth's Starter Mix. Most hardware stores carry it, so finding it should be a breeze. A good second choice is Bu's Compost Seed Starting Mix.