To construct a mental Hlidskjalf and activate it for use in Norse pagan Asatru, one could follow the following ritual:

1. Begin by creating a sacred space, either outdoors or indoors, where you will perform the ritual. This space should be quiet, peaceful, and free from distractions.
2. Place a candle, incense, or other offering on an altar or table, to symbolize the presence of the gods and the sacredness of the ritual.
3. Begin the ritual by invoking the gods and goddesses of Norse mythology, such as Odin, Thor, Frigg, and Freya. This can be done through chanting, prayer, or other forms of invocation.
4. Next, focus your intention on the construction of the Hlidskjalf, and imagine or visualize the creation of this powerful device. See it as a throne or seat of power, upon which the gods sit and observe the nine realms.
5. Use the runes or other symbols of Norse mythology to activate and control the Hlidskjalf. This can be done through visualization, meditation, or other spiritual practices.
6. Once the Hlidskjalf has been activated, focus your intention on a specific destination in time and space, and use the Hlidskjalf to travel there. Imagine yourself seated on the Hlidskjalf, observing events and occurrences from the past or future.
7. After you have completed your journey through time and space, return to the present moment, and give thanks to the gods for their guidance and support.
8. End the ritual by extinguishing the candle, incense, or other offering, and closing the sacred space.

This ritual can be adapted and modified to suit the individual’s personal beliefs, practices, and experiences. It is important to approach this process with an open mind and a willingness to experiment and explore, in order to discover what works best for you.