Astral travel can be defined as having two aspects.  One of discovery and the other of learning or inspection or insight. 

The first travel mode, that of discovery, is when the soul allows whatever to happen as it wanders.  It may or may not obtain the astral plane. 

In the second mode, with help of the subconscious mind, travel can be directed.  Just as in a seed thought meditation, a seed is planted so one must plant a seed thought before travel.

The Process

The process may go something like this. 

As you lay in bed you first feel comfort and quiet.  Plant in the mind the expectation of travel by visualizing weightlessness or floating or levitation and how nice it feels.  As you do this, plant also the thought that you will travel to the astral plane as you levitate or feel weightless.

After success in obtaining the feelings of weightlessness or levitation during sleep, one may assume that the soul has traveled but the mind does not recall. 

The next step, once this has been achieved, is to add the thoughts that while levitating or weightless or traveling, the mind is open and receptive and remembers. 

Once you recall a travel experience (you will recognize it by its unusual and somewhat nondescriptive feelings) but you will feel that you did travel; perhaps not convinced at first, but with practice you become sure. 

The next step is to plant a seed thought so that once you obtain the astral plane, your subconscious can direct the soul in its mission to find insight to bring back to the mind.