There are many styles of seed starting. You should pick one that suits your inclinations, or try several on a small scale and perhaps be surprised by what turns out to be easy or pleasurable for you.
I like to watch and fuss over my seedling trays. As soon as they emerge, they need the plastic cover removed. 100% humidity encourages damping off (soil fungus that attacks the seedling stem).
Once the seedlings emerge, I don't have enough room to pot them all up, so I wish I had a greenhouse or cold frame to harden off the seedlings.
1. wintersowing -
Start seeds in small covered containers or cut-up milk jugs outdoors in winter or early spring. Just leave the containers outdoors in the shade so they don't "cook" some sunny Noon-time. When the weather warms up, the seeds will sprout when the warmth INSIDE the containers meets their needs. They will need some hardening off when you take them out of the jugs.
This is an easy way to provide cold, moist stratification for perennial flowers that need that.
Many or most WS-ers sow so thickly that they can only break up the seedling mat into "chunks" and they plant out 5-50 seedlings in each chunk. That also works better with flowers than vegetables.
But you should ask someone who does a lot of WS for the tricks of the trade. Many people say passionately that this is the easiest way to start seeds.
2. seed flats: pricking out & potting up -
Broadcast many seeds onto a shallow flat. You can fill an 11x21" tray with one variety of seed, or use pots as small as 4"x4" to start many seedlings rather close together. There are 11x21 flats available with 20 small "rows" if you also consider that a "seed flat".
Cover the flat with plastic to hold in humidity. Maybe provide a heat mat if the seeds need warmth. Provide light or darkness according to the seed's dormancy requirements.
You have to "prick out" the best seedlings before their roots get big enough to tangle tightly - probably when the seedlings only have cotyledons or 1-2 pair of real leaves. Then "pot them up" into larger pots and grow them on for a while before hardening off and transplanting.
That's what I dislike about this method: you have to untangle seedling roots and that gives me the heebie-jeebies. I'm afraid that I'll kill the seedlings by touching the "tender" roots or just exposing them to light and dry air.
Others tell me that I'm silly: that they just rip those seedlings out casually and pot them up and plenty survive unharmed.
I think "flats" are the best way to start LOTS of plants. They take up almost no room at all until you prick them out and pot them up. You might start seeds indoors, under lights, in flats, until they are ready to pot up. Then move the 4" pots out to the greenhouse and natural light after they adapt to being potted up.
3. small cells - plug trays, propagation trays, inserts, or trays of small pots.
I love "propagation trays", "plug trays" and "inserts". You start just one seedling per cell and never have to untangle any tiny roots. Just pop out the root ball and plant it whole, no "pricking out".
Inserts are flimsy plastic that can be torn apart into "6-packs" or "4-packs". Big box stores sell small plants in these "inserts". An 11"x21" insert tray would have 72, 50, 32 or some other number of inserts. The inserts are very flimsy so you need to support them with a "1020 tray", usually with no holes, so they hold water. I re-use inserts, usually the 72-per-tray size. I think most people throw inserts away after each use.
"Propagation trays" or "plug trays" are much more sturdy and support their own weight. I plan to re-use my prop trays forever. You can use plug trays without anything under them, if you're willing to have muddy water drip on your shelving.
(Plug tray = propagation tray = prop tray, as far as I know. maybe there is some subtle distinction.)
They usually have smaller cells and more per tray, so you can start nearly as many seeds as you can with flats. I have 11"x21" plug trays with 50, 72, 128 and 200 cells. You can buy plug trays with 288 cells! The tray has 12 rows by 24 columns.
The advantage of inserts and plug trays over flats is that you start 1-2 seeds in each insert or cell. Then you cut all but the best seedling in each cell. Let the best seedling grow in each cell until it is sturdy enough to pot up, or until it is almost root-bound in its cell. Then pot out
If you prefer to avoid "potting up", you can transplant directly from plug trays into the soil. That plan goes best with largish cells, like 72 or 50 per tray. You can find inserts with as few as 18 cells per tray - around 3.5 inches each. I tend to use those only as mini-flats, sowing multiple seeds per insert cell and forcing myself to prick them out young.
4. start in pots
You can start seeds directly in small pots or large inserts. Most people plant multiple seeds per pot and keep only the best ones. If they pricked out some of the rejected seedlings from the pot, and potted them up to keep them, then technically the small pot was really a "small seedling flat".
I think that any of these schemes could be done outside in a greenhouse, as long as you check them frequently. My inclination is to start seeds indoors under lights, fuss over them obsessively, and then move them outside when I need more room in my seed-starting shelf.
5. Deno Method: germinate on a damp paper towel in a plastic baggie
You can start seeds on damp paper towels (inside baggies to keep the humidity high). As soon as the root emerges (the radical), plant the seed shallowly, root down, in a 4" pot. let it establish itself in soil before hardening off.
This can be combined with refrigerator stratification, and it works well for seeds that might need weeks or months to germinate. It's easy to do this method very cleanly or even with near-sterility, so the risk of rotting is less than in soil.
