You may start your GTD implementation by following one of two approaches:

1. Dealing with the most mundane, ground-floor level of current activity and commitments first, to later discover higher horizons. This is what we call Bottom-up Approach.
2. First uncovering personal and corporate missions, then defining critical objectives, and finally focusing on the details of implementation. This one is known as Top-down Approach.

The "bottom-up approach" is usually more effective than the top-down one, as most people are so embroiled in commitments on a day-to-day level that their ability to focus successfully on the larger horizons is seriously impaired.