If you are doing pastel colours I think it's really important to get the hair to a clean level 10 first. Then I would either tone the hair to clean out any remaining warmth, rinse and re-tone with the pastel colour but sometimes you can mix them in with the toner instead.
Pastel colours are always going to fade quickly, but if you do something that will fade back to a nice clean blonde afterwards most people are happy, otherwise they will need to have regular toners or use coloured shampoos.
Products wise I personally tend to use demi-permanent colour, it's more gentle than permanent and will often give you a more even result that direct semi's, which can also grab uneven on porous areas or fade off tone like green. But direct semis are great for more vivid tones. Demis are good for more softer, dusty or metallic tones.
I would usually take a clear colour and add a few centimeters of a darker violet/blue/red to dilute it down to a lavender/pink/peach/lillac etc. Or you can use premixed pastel toners like wella instamatic.
