As all magical things do, it all starts with an act of imagination. It’s similar to the way a child can turn a cardboard box into a spaceship or a race car. If you could look through their eyes, you would see that the box is a spaceship or a race car because that is what they are seeing.
Astral travels are the same way. If you can sit on your couch but see yourself on a beach with your mind’s eye, and hear the seagulls, and feel the breeze on your skin, then you have all you need to enter a square. All it takes is practice.