Where most beginners fail in attempting to start seeds is in not taking the trouble to prepare a proper soil. They are willing to take any amount of trouble with watering and heat and all that, but they will not fix a suitable soil. The soil for the seed box need not be rich, in fact it is better not to have manure in it; but very porous and very light it must be, especially for such small seeds as most flowers have. Such a soil may be mixed up from rotted sod (or garden loam), leaf-mould and sharp sand, used in equal proportions. If the loam used is clayey, it may take even a larger proportion of sand. The resulting mixture should be extremely fine and crumbling, and feel almost "light as a feather" in the hand. If the sod and mould have not already been screened, rub the compost through a sieve of not more than quarter-inch mesh--such as a coal-ash sifter. This screening will help also to incorporate the several ingredients evenly and thoroughly.
While we provided holes in the seed box for drainage, it is best to take even further precautions in this matter by covering the bottom of the box with nearly an inch of coarse material, such as the roots and half decayed leaves, screened out of the sods and leaf-mould. On the top of this put the prepared soil, filling the box to within about a quarter of an inch of the top, and packing down well into the corners and along sides and ends. The box should not be filled level full, because in subsequent waterings there would be no space to hold the water which would run off over the sides instead of soaking down into the soil.
The usual way is to fill the boxes and sow the seed, and then water the box on the surface, but I mention here a method which I have used in my own work for two years. When filling the box, set it in some place where it may be watered freely, such as on the cellar floor, if too cold to work outdoors. After putting in the first layer of coarse material, give it a thorough soaking and then put in about two-thirds of the rest of the soil required and give that a thorough watering also. The balance of the soil is then put in and made level, the seeds sown, and no further watering given, or just enough to moisten the surface and hold it in place, if dry. The same result can be obtained by filling and sowing the box in the usual way, and then placing it in some place--such as the kitchen sink--in about an inch of water, and leaving it until moisture, not water, shows upon the surface. Either of these ways is much surer than the old method of trying to soak the soil through from the surface after planting, in which case it is next to impossible to wet the soil clear through without washing out some of the small seeds.