Ensure the sink is clear of all food remains and washing up
Make sure the temperature is reasonable
Wear something which you don't mind getting covered in water
Don't use excess soap – it makes it even harder to get it all out
Ensure eyes, mouth, ears and nose are closed – soapy water leaking into all 6 holes at once is NOT nice. (not entirely sure how you could close ears, but never mind….)
Have someone nearby to help when you realise that without them you will NEVER get all the soap out
Preferably this person shouldn't be your dad….
Please note: Kitchen sinks are NOT designed for washing hair.