Alright. You really should have had all of your growing medium set up and ready to go beforehand but regardless.
If you want a primo soil with all the fixings, check out the hydroponics stores on eBay or check online for a retail hydroponics outlet in your area. If not, you can mix your own. For my project I'm using the following mix:
Peat Moss, Perlite, Blood Meal, Worm Castings, and building sand.
The peat moss, perlite, and blood meal are available at Canadian Tire or other general garden center. The worm castings I bought online at high tech garden supply (google them.) The sand is industrial level so I got it from an old abandoned construction site near my spot.
You should place the seeds in the soil once the root has grown a 1/4" to a 1/2". Place the seed in the soil root down and gently pack the remaining soil around it.
Water it in with a good deal of distilled water. If you haven't already, invest in a pH meter. Ideal water should be a pH of 6.5-6.8. I use collected rain water with a pH of 6.5, however you should check the pH of your water before you use it as the pH of rain water varies by the living environment.
Get a small container to help propagate the roots or grab some rockwool cubes to start the plants out from seed (also available at your local garden center). When you are ready to transfer to your grow box, take the seedlings and place them in 4 liter pots (or relative size) and cover around the plant with soil.
There are nutrients in the blood meal (12-0-0) and depending on the particular numerical value, certain nutrients help out with certain stages of growth.
Marijuana uses up a great deal of nitrogen (N, or the '12' in blood meal) in its life cycle, so supplementing with a high-nitrogen fertilizer is a good idea. However, I recommend holding off on nutrient solutions until one month into plant growth.
Hope this helps.