The basic technique is pretty simple: Get into a meditative state and visualize an empty room. Add whatever furniture you want to this room. Carve a magic circle in the floor. Add an altar. Have rings of candles and tapestries, whatever ritual tools you want. Whatever you can imagine you can put in your temple. It’s a lot easier to evoke a demon within a blazing circle of flames in the astral than it is in the physical, for example.
Try to spend some time in your temple every day. The astral is always in flux so you will find that your temple will change itself each time you visit so you need repetition to solidify it. Generally, it will take about a week or two before it will remain as you envision it. Once you have reached that point, you can then begin to perform your rituals in there. When you do, try to see yourself from the first person perspective. When you pick up your dagger, for example, try to see it in your mind from your own point of view instead of from the outside like a movie. Also make sure you include all the senses, not just sight. Smell the melting wax of the candles, feel the cool air blowing in. taste the electricity in the air, feel the heft of the dagger in your hand.
Here is a tutorial: