If you hope to get pastel hair, you must have an already even blonde hair colour.
You should colour the base colour of your hair to a shade lighter. (bleach)
Mixing ratio: 1 unit of hair dye + 2 units of volume developer
Apply the colour on the whole length of your hair and leave it on for 15 mins.
If you want to end up with a nice even colour, work the colouring cream through your hair every 5 min.