The following quantity would cover a 8" - 10" cake. For smaller bakes halve the ingredients.
If you want to colour your fondant add the colour to the marshmallows before the icing sugar is added. Of course if you wanted pink fondant, buy pink marshmallows!
If you want a flavour add at the same time.
250gms White Marshmallows
500gms Icing Sugar
1 Tablsp Water