Astral projection is not always necessarily an out-of-body experience. It is a method of visualization, employing a deep meditative state.
While some people do achieve an out-of-body, fully immersive type of experience with relative ease, these cases are relatively rare.
The most successful method(s) of experiencing the astral are through guided or deliberate meditations of moving through an environment, into a means for transport to a different environment. For example, a person may visualize walking through a peaceful forest, to a lake below a cliff, then through a waterfall on the other side -- or even imagine ascending the waterfall. At a point, ideas or characters may present themselves at the top of the waterfall.
The astral is different for everyone. Since the images are all filtered through the practitioner's own mind, imagery will be specific to its meanings for that person. If someone wants to visit the same 'location,' they may have a very unique experience compared to someone else, though there might be correlation of some concepts and elements.
It is important to know that during astral projection, the spirit does not leave the body, regardless of what may be experienced. A person's awareness is basically extended to the Astral, as opposed to the material or mental. It is still important to use protection, wards, and the like, as defense is a good idea when paying strict attention to something else.
All of our bodies are interconnected and always connected and communicating. In order to develop your astral projection abilities, you need to tap into your astral body awareness. This is easier done for some people via using a certain technique of repetition completed by the physical body. This is usually the easiest way for very grounded people, to start letting go of their logical mind. I used to use Tai Chi, yoga and weapon training, if it was harder or me to let go due to anxiety. Training your physical body seems to put it in ease to face challenges. A physical body under physical stress an facing direct danger will not let go, unless you start to die or you have natural ability to berserk or switch to flow state.
After that, you can use meditation. Meditation is actually a form of mental development or training. It is hard for most people to train their mind, and the motivation factor is not always there. There are many type of meditations like mindfulness, concentration, loving kindness and compassion, tranquility and last one is insight.I would suggest you try them in this order. Insight one is hardest ,but then your awareness starts to expand wider. During meditation, you train your brain to function in different mode, which ability we lost somewhere around the age of 5. It takes time to achieve that.
As prsona mentioned, some people can naturally enter fast very expansive state of awareness on all levels. This is because they never fixed their awareness because of the specific type of brain they were born with, or they had out of body or near death experience. Even so, training of the mind helps me to control this expansion. Emotional control helps with avoiding the dark places on lower planes of existence. Consciousness uses emotions and connectivity as a direction to where to expand to. It also takes very strong willpower and discipline to not allow these other planes to affect you and well-being on long run.
Some people can astral travel in their sleep naturally, without realizing it or ever working on it. Others can use any state of being and switch between states naturally. However, natural talent into something does not equal instant mastery. It means you end up in big trouble and do a lot of mistakes. Until, you start to gain mastery by learning the harder way, but the faster one. Slow and steady means less trauma and trouble. I lost count how many times I ended up in very bad place. Few beings had to drag me down to my body, after I was lost for 24 hours (on the physical plane). I had no perception of time on the other. I was indeed very troublesome case for my spirit guides.
Learning these skills takes time and facing challenges. People just wrongly assume, that they can master the skill for 5 minutes a day for week. It is actually a lifetime skill to master. Even few lifetimes for some souls. Any practice is better than nothing, but the less practice the further way the master inside you is. It is also about self-mastery. If one can master any aspects of themselves, can master each of his bodies. When these bodies work in harmony, mastery is achieved.