When you first plant the seeds, you should also top water once with a fine mist water sprayer. The soil should be well watered throughout but not soggy. Be careful that your mini greenhouse isn't too humid. Wipe off any beads of condensation that form on the containers lid. Also be careful that the temperature isn't too hot, as this can cook the seedlings.
Many species germinate within a few weeks.
When the seedlings first appear, they look like tiny green spheres. After they have sprouted, replace the Tupperware lid with a piece of stretched muslin secured with string or a rubber band. This will allow air circulation, which can be increased by placing a fan above the container. Adequate air circulation is essential as all green plants require plenty of CO2 to grow. Seedlings are more sensitive to light than mature plants.