Commercial potting soil is often difficult to wet. After it is placed in the container water it several times the day before you actually sow any seed. Water passing though the drain hole is a good indication of moisture content, but also dig into several containers to see if all the material is saturated. With good drain holes in the container, there is no danger of over-watering. Once the seeds have been sown in the mixture, the container is placed in a plastic bag to retain moisture.
Directions often suggest placing the container in a bright location next to a window.
In such a location the seedling container may get too cool for optimum germination. Particularly at night, a container set next to a window may get much colder than you realize. Nearly all garden seeds need a minimum temperature of 70 degrees F both day and night until the seedlings are up and well established. Pepper, eggplant and tomato seeds require temperatures as much as 10 degrees higher for good germination. Vine crops like cucumber, muskmelon, squash and watermelon germinate very well at temperatures close to 95 degrees F. Cool germination temperature is probably to greatest cause for failure for many gardeners.