So you've built your cold frame, now what?
Line the bottom with paper feed bags or landscaping fabric (this is to suffocate the grass and weeds underneath. Add the soil. I use mushroom compost which I buy from Kevin Davis in Tulsa (918)280-0717 they will put as many front end scoops as you can carry for a really great price. They also have a garden mix which is mushroom compost and sand. One load of garden mix mixed with 1/2 bag of peat moss and your ready to roll. Now if you cant get to Tulsa and don't have a pick-up, instead buy about 5 bags of some kind of nice compost. Mix it with one bag of sand and 1/2 bag of peat moss. If you can find mushroom compost or cotton burr, that works really well. Just don't use miracle grow only because seedlings have a bad habit of dying off just all of a sudden like. I think too much nitrogen.
The seeds I like to use for a baby lettuce mix is black seeded Simpson (its a green leaf) red oak leaf, arugula, spinach and bulls blood beet (just for the nice red greens). Plant in rows that are 2" apart and just kind of sprinkle the seeds into a little trench (1/4" deep) that you make with a pitch fork or something. cover with soil and water. Keep the soil moist until seeds germinate. Right now you can leave the lid off at night unless you need to keep your cats out, just don't forget to take it off in the morning. If you plant now your seeds should germinate in a few days (3-5). The cooler it gets the longer the seeds take to germinate.
Good luck, contact me if you have any questions.