Generator synchronization to grid and angle difference
If the frequency is in fact identical, then the phase angle will never change. However, this is not what is meant by that sequence of instructions, nor what happens in practice.
Getting the frequencies equal means nominal frequencies, getting it fairly close. With an unsynchronised generator, any slight change in drive power will alter the frequency slightly.
While the frequency is close, the phase will be changing slowly with respect to the grid. You observe the phase meter, and close the contactor when it's less than some specified tolerance.
If the phase is changing too fast, then the frequency is too far from the grid frequency. Adjust the frequency until the phase is changing slowly.
If the phase is wrong, and is not changing at all, then adjust the frequency until the phase is changing slowly.