Right now we have 5 flats under lights, mostly planted with heirloom tomatoes, peppers, brassica crops, herbs, and cucumbers, zuchini, and squashes germinating under our lighting setup. Only five flats, but that represents between $200 to $250 worth of seedlings, and these are all much higher quality seedlings and much more interesting varities of of plants than anything you are likely to be able to get at the local seedling sources - especially the heirloom tomatoes.
Here's how we prepare the flats. First, since I am reusing some of last years flat trays, I sterilized them by washing them (in the bathtub) and then spraying them with a mild chlorine solution, of about two tablespoons of chlorine beach in a quart sprayer bottle filled with water. Spritz spritz spritz all over and then let it sit for an hour. Careful, the chlorine is pretty nasty - but it sterilizes the tub too.
I like to use jiffy strips, preformed peat cups, to replace the molded plastic panels of individual seed starting cups that come with teh seedling flats when you buy them. BTW, I buy these tray sets at Lowes, 3 trays for 9 dollars, which is pricey enough but they can be reused, and its still way cheaper than buying the seedlings.
(And remember, last year all those seedlings were pre-infected with the tomato blight, which killed everyones tomatoes. So starting our own seedlings protected us from the worst of the blight, and we still had a decent harvestm altho all the plants did eventually wither, brown, and succumb.)
Here's the rest of the proceedure for filling the trays, in pictures.
The basic starting elements of a seedling flat. The plastic tray, the individual containers for the seedlings, in this case preformed peat pots, and some potting soil.
Read more about Starting seedlings - step 1 - preparing the seedling flats, filling peat jiffystrips with potting/starting soil