It's early February and while it's warm out right now, it's still a little too early for seed starting. Last frost here in the Willamette Valley is the first week or two of April. From my own experience and the particulars of my garden I will put out my coolest weather plants in late March. I have plastic sheeting to cover the plants if there is a late frost expected, and my beds are all raised so the soil tends to warm up faster than in-ground beds.
Far more of an issue in my garden is that the fence along the back of each bed is south facing. Right now we can have a beautiful sunny day and my beds are completely shaded, as well as several feet past them onto the patio. The fronts of the beds won't see direct sunlight until mid-March, and the backs won't until late April. I can start seeds indoors though and put them out in mid- to late March. The only issue with this is trying to find a balance between when to plant versus how large the plants will get by the time they can be moved outdoors. My small greenhouse will keep them safe a little longer, but they will become spindly and weak quickly and exposed to very high temperatures in the greenhouse when the sun is out. The little standing shelf style of greenhouse is useful for the transition between indoor growing and outdoor growing, but it really doesn't have any temperature controls and heats up very fast.
I generally figure about 10-20 days of indoor growth. The first ten days are just to allow for sprouting, the next ten are to allow at least one but preferably two or even 4 sets of leaves to develop before transplant time. Most cool weather plants shouldn't have trouble with this; others, such as celery, need much longer to develop.
I also don't have any south facing windows or any grow lights currently, so if I plant seeds too early they will get lanky searching for light long before it's warm enough for them outside. The only windows in my house that face south are in my kids' bedrooms, and that's just not a practical place for plants to be.
My best bet for starting brassicas, lettuces, spinaches, and other cool weather crops is to plant them around the last week of February to the first week of March, then move them to the greenhouse in mid-March and wait for the weather to clear for transplanting. This is one of the reasons for keeping notes on your garden. Last year I started things inside the first week of March, then directly transplanted them at the end of March because I didn't have the greenhouse yet. They were very spindly after a month in the house, and i was forced to put them directly in the ground a little too early because the greenhouse was tipped over in a Spring storm. I learned to weigh it down with a cinder block to prevent that happening again! The plants struggled outside until the sun finally started to reach the garden beds in April, they did well after that though.
This year I think I can get a head start by starting just as early indoors, then moving things to the greenhouse, and holding off to transplant until April.
One thing I do plan to change is the size of pots I will be starting seeds in. Last year I used the tiny 2" peat pots from a seed starting kit. This year I will be starting seeds in 4" pots, this will save me from having to transplant them before they outgrow the 2" ones. Since I'm not running a nursery where seed starting space is limited it makes more sense to me to save myself the step of switching up to bigger pots.
You can start seeds in anything, peat pots, plastic pots, egg cartons, or paper pots. Anything that is roughly the right size and will hold dirt and water, just make sure it has some drainage holes in the bottom. You will also need a tray of some kind to catch water runoff; plastic trays from seed starting kits are reusable for this year after year, cookie sheets with sides work too, or a mesh screen over a bucket even. My dad found a collection of old school lunch trays at a garage sale many years ago that work great for all sorts of things, including seed starting trays.
My brother found a very cheap solution for pots as well, that also protects against transplant shock. Newspaper pots! Fold newspaper origami-style into a pot shape, plant your seeds, watch them grow, then plant the whole pot into the garden. Here is an excellent video showing how to make these. If your origami skills are lacking you can also try this method.
So you have a tray, and pots, now you just need some dirt. The one thing to be careful of for seed starting is not to use old, used dirt from the garden. Normal garden soil is full of weed seeds, fungus spores, insect eggs, etc. that you just don't want in the pots with your seeds. You should use potting soil, seed starting soil, or if you prefer a less expensive option you can use peat moss and give it an occasional dose of compost tea once the seeds sprout. Peat moss can be bought for a few dollars a cubic foot, which is more than enough to start all the seeds you need. It retains water well so won't dry out between waterings and is soft and loose, allowing roots to spread out and grow better. Peat moss doesn't have any plant nutritional value though, it's like the potato chip of soils. Spritzing your starts with compost tea once a week will help supply the nutrients they need to grow healthy in a peat soil, and prevent them from damping off before they are transplanted.
Plant your seeds, smaller things like spinach can be planted 2 or 3 to a pot, larger plants should be in separate pots. Label each pot, see tip below. Find a sunny (or somewhat sunny) spot in the house where seedlings will be safe from kids and pets (my cat ate my first lettuce plantings last year). Cover trays with plastic wrap, or the plastic domes that come with seed starting kits to help retain moisture and heat. Remove the plastic covers once the seeds have sprouted to prevent mold though! And wait for your sprouts to come up.
Make sure to note what day they were planted and what day each type of seed sprouts. If you have sprouts of everything but lettuce after 10 days, you should probably plant new lettuce because it should have been the first thing to sprout. Again, this is when taking notes is really useful.
Tip: Don't write labels on the paper pots, they will smudge after watering. Instead, you can use plastic knives from party packs of plastic silverware (because you always end up with leftover knives, yet need more forks and spoons) and a permanent marker. These work great; they are sturdy, don't smudge, and it's a nice way to reuse something that would otherwise just keep stockpiling in your cabinets. Another option that I found at Backyard Homesteader's YouTube Channel is using window blinds as plant tags, plastic blinds can be found for a two to three dollars at most stores that sell housewares, then cut up into small tags to label plants with.