Generally, you should focus the ITIL implementation based on the following elements (your milestones):
1. Strategy – Define: where do you want to be? Who are your customers? Which market do you serve? How do you intend to achieve those goals?
2. Customer journey – define your activities (and related resources) throughout the customer lifecycle
3. Products and services – define activities related to your products and/or services. Define related methodologies.
4. Operation – once you have your products/services – define needed activities to support and maintain them. This also includes related resources (and their capabilities) as well as respective value streams
Measurement and metrics – define (and implement) appropriate measurements and metrics in order to ensure efficiency in service delivery