Sounds like an exercise in visualization. Just get real quiet and relax to a meditative state and put one together. Know your deity or entity and what they like and then build it and make a sacrifice or offering of devotion in your mind.
It is considered a mental construct, and can be absolutely valid. The more effort you put into it, the more real it becomes in your mind's eye, the better it will be for you.
Some people believe that anything mental used for practicing spiritual things is done on the astral, as the astral intersect the mind.
Another practice is the creation of an astral temple. Some people insist that one cannot be used until its creation is complete, but I can disagree with that. Build it as you may. It is a working space in the dimension of the mind, so it can be and do whatever you choose.
You can construct a sacred space while you are meditating. Visualize every detail, the shapes, the textures and weight of the objects. The style of the room, the furnishings, the colors. You can make it as plain and simple or elaborate as you like.
I agree with Prsona and wouls also agree that visualisation is an astral skill- but people use different definitions for different things.
I also agree that an astral temple can be used before 'built' as your work will go on you will also find you astral temple will change or rather, you will change it as your understanding of magic and the universe grows as well as necessity for personal work. Mine changes quite regularly depending on what work I am currently undertaking in it.
I believe a physical temple is better, however, in this day and age a permanent personal temple is a hard thing to keep.
Keep in mind that you needn;t have a temple to a deity. Your body is a temple in itself, your mind too. Keeping a deity ever present in your thoughts can be a powerful devotional act in itself. People can pray at an altar for 12 hours a day, but unless their deity is in their mind outside of the temple their efforts will be in vain.