It is great that you would like to start to improve your health and lifestyle.
NEVER! You have several options:
You can start at week 1 and make your way through the changes.
You can just start now with the current weeks change and continue with us, and then carry on with the changes you missed.
Either of these will be the best, because when it comes to creating new habits and getting them to stick t is by making small changes.
Put a checklist on your fridge to remind you what to do until your new healthy habit is formed.
Alternatively, you could try combining a couple of changes a week whilst you catch up, so an extra glass of water plus an extra vegetable.