The time average of a periodic quantity $Q(t)$ is, by definition,

$$\langle Q \rangle=\frac{1}{T}\int_0^TQ(t)\,dt$$

where $T$ is the period.

For Keplerian orbits, it is usually easiest to change the integration variable to the angular coordinate $\psi$ and express all quantities being integrated in terms of $\psi$.

The exercise thus consists of doing a variety of angular integrals. There is a typo in one of the results.