There you can find all the info you need on seed starting and frost dates.
It's important to start seeds in the right mixture. You can buy medium that says it's seed starter but I just prefer to mix my own. Here's what I use in my mixture -
Then I select a suitable tray depending on how many seeds I will be germinating. In the case of the impatiens the packet states 25 seeds. You need to plan how much space each seedling will need in the tray so that they are not overcrowded. Also you need room to prick the seedlings out of the tray when the time comes.
Now it's time to get your hands back into the soil. I grab about a handful of each part of my mix as you can see here -
The number of handfuls required will be determined by the size of your tray. Then dump the mix into a bucket and (VERY IMPORTANT) spray with water. You don't want to breath any vermiculite or perlite dust! Spray it down as you mix it up.
As you create your seed starter pick out any large sticks or other debris in the soil mixture. Then fill the tray and spray some more water on to just dampen it. Since I am starting impatiens and they are very small seeds I put a thin layer of vermiculite on top. It will help to hold the moisture but won't smother the seed.
Now carefully open the seed packet and make a fold in the flap. Shake a few seeds toward the end of the flap. Place each seed in the seed starter medium. In this case I put 5 seeds in 5 rows, and I think they gave me a couple extra so I placed those between rows.
The seed packet recommends that you "plant the seed in a sterile planting mix but do not cover as light aids in germination. Bottom water or mist to avoid covering the seeds".
Here are the hydration tools for applying moisture. -
Two very important factors in successful germination are moisture and temperature. Until the seeds begin to sprout, keep them moist. As for temperature don't assume that because you keep the ambient air temperature at say 70 degrees that the soil temperature will be the same. The same way that perspiration cools the body, evaporating water will cool the soil. A couple years ago I invested in a heating pad for the seed flats and last year I purchased a sensing probe and thermostat for the germination trays.