I've learned that though I can follow the same basic principles for tomatoes, groundcherries, and tomatilloes, I have to use a different approach for the eggplants and peppers. In short, I start peppers and eggplants in a folded paper towel, dampened with chamomile tea (see rationale below), and place them in sealed plastic bags on a heat mat until they sprout. It can take some species of peppers and eggplants weeks to germinate. Once most are showing roots and the first glimpse of leaves, I carefully plant the sprouted seeds in scrubbed-clean six-pack pots and grow them inside, under lights. Peppers and eggplants need a lot of fertilizer, much more than tomatoes, so I begin fertilizing the seedlings with a very dilute fish emulsion every time I water them until they go in the ground, when it is reliably warm.
A great description of this approach with helpful photos is available here, but I do a couple things differently: I don't have coffee filters around, so I use paper towels, and I carefully cut the paper around the seedlings to remove them, rather than trying to lift them off and break their roots.
My tomato (tomatillo and groundcherry, as well) method requires a lot less special equipment, and reliably gives very good results. I have based it on the information I learned from wintersown.org, a helpful, free resource as well as a place to get free open-pollinated tomato seeds! Tomatoes and their close kin need less heat and less fertilizer than peppers and eggplant to get strong; they can handle more fluctuations in temperature, and in fact, seem to benefit from them. The makeshift greenhouses wintersown.org teaches how to make are a perfect fit for tomatoes.
I start with a large batch of chamomile tea. Drinking chamomile tea gives me a headache, and handling it makes my skin itch like mad, but chamomile works exactly the way I need it to here. It is a natural fungicide, and therefore helps to prevent the dreaded "damping off," when fungus attacks a just sprouted seedling, toppling it over like a felled log.
I use this batch of tea to dampen potting soil in a large bucket. I've tried using potting soil made just for seed starting, and I've found it a wasted expense. I had no more success with specialized seed starting soil than I do with a high quality, fine textured soil.
Here comes the fun part. I start making mini-greenhouses with plastic bottles I've scavenged from work and home. Using an awl, I punch holes in the bottom, and with a sharp bread knife, I cut the midsection of the bottle most of the way through, leaving a hinge.
I lightly tap the tea-dampened soil into the bottom of the greenhouse. I don't want it to be too dense, as there needs to be plenty of air and room for the roots to fill. Before I gently slide the seeds just under the surface of the soil with the end of the awl, I place a few of them on the surface of the soil, spaced about an inch apart.
Finally, I flip the top part back over, screw the lid back on the bottle, and tape the greenhouses shut with their labels. Outside they go, where they live on the patio where they get morning sun but not blazing hot afternoon sun.
Once most the seeds sprout, I remove the screwtop to allow ventilation, as the seeds don't need to be quite so warm once they've gotten going. I monitor moisture, spraying water through the top opening if needed. When the first true leaves grow, I slice carefully through the tape closure to allow the greenhouses to prop themselves open a bit. After there are two sets of true leaves on most of the seedlings—that may vary by variety and some varieties will be ready to pot up before others—I use a utility knife to slice the top completely off; then I slide the knife down the side of the base pot and gently release the seedlings. Each healthy seedling I pot up in its own 4" pot.
As the days get warmer, I move the seedlings to spots and that are gradually more exposed with more sunlight and more temperature fluctuations, until they finally are living in a spot similar to where they'll reach adulthood.
Come spring, there are seedlings to plant and spares to share. I can't wait for that green tomato-leaf smell.