Creating the Astral Sanctuary (or mental mansion/palace) may be very easy for some, and extremely difficult for others. The processes may vary, but there are some useful tips. Here are a few, along with my way of building the sanctuary.
Learn to meditate. Learn to do so in all environments, until it is effortless to meditate in almost any situation. This is the most difficult part. Many may need several years to get to the point where clearing the mind takes little effort.
Exercise your imagination, and visit many places to see what you like the most. For example, maybe one enjoys cool, sunlit beaches where the wind blows gently and seems to take away all negativity away. Then close your eyes, and picture a place like that, or with whatever characteristics you enjoy. Lay down, if it helps, or assume a relaxed yoga pose such as corpse pose. Visualize all of your favorite things. Take them all in your hands. Then visualize yourself on a flat, dark plane. Use everything you are holding and make it your sanctuary. Remember, on the plane where you are, the laws of gravity and physics do not have to apply.
Be sure to meditate a bit before trying any of this. A cluttered mind will result in an incomplete sanctuary that fades very quickly. You need to be able to focus to build the palace of your dreams.
If you should become very skilled at this, then you will be able to call up your sanctuary and see it around you, augmenting reality, and allowing new and more powerful spells to be made and cast from within.