When he wishes to descend he draws around himself a veil of the matter of the lower mental world, which we call his mental body.

This is the instrument by means of which he thinks all his concrete thoughts--abstract thought being a power of the ego himself in the higher mental world.
Next he draws round himself a veil of astral matter, which we call his astral body; and that is the instrument of his passions and emotions, and also (in conjunction with the lower part of his mental body) the instrument of all such thought as is tinged by selfishness and personal feeling.

Only after having assumed these intermediate vehicles can he come into touch with a baby physical body, and be born into the world which we know.

He lives through what we call his life, gaining certain qualities as the result of its experiences; and at its end, when the physical body is worn out, he reverses the process of descent and lays aside one by one the temporary vehicles which he has assumed.

The first to go is the physical body, and when that is dropped, his life is centred in the astral world and he lives in his astral body.
The length of his stay in that world depends upon the amount of passion and emotion which he has developed within himself in his physical life.