Factors affecting seed germination: practical

Some seeds (e.g. cress seeds) will germinate on cotton wool in the right conditions. Planning an investigation and writing a method for factors affecting the germination of seeds on cotton wool. Deciding which variable to change to become the independent variable and what values to investigate. Deciding which variable to measure to conclude on the effect (the dependent variable). Deciding which variables to keep the same to ensure a valid conclusion (control variables).