In essence you can start setting up the lean production system right away. But, as you can also read in the roadmap section, this is a project of creating a culture, and it is a long-term project. Applying the lean production system slowly, by absorbing it, will make your employees and you steadily happier.
There is no harm in immediately improving points where the chance to improve is clearly visible. But as the lean production philosophy spreads, problems will become visible, the benefit will grow as you solve them, and you will start to become more successful.
For example your costs will fall, your profit will rise, your output per person will rise, scrap will fall, stocks will fall, and you may even give up building a new building. How? By taking the first step right away.