To reach the final stages of yoga, it is absolutely necessary to develop the power of control and concentration stage by stage.

There are eight stages.

First stage of control is Yum (as in Yummy)
Harmlessness; love for all
• Honesty
• Non-theft
• Kindness
• Forgiveness
• Assertiveness
• Moderation in eating
  Second stage of control is Niyum
Contentment
• Purification of inner and outer body
• Charity
• Modesty
• Reflection
• Focusing on the positive
• To study spiritual books

Third stage of Control is Aasans
Body control thorough posture. The body should be purified, vitalised and well-controlled by hatha yoga, and in that state of the body, concentration will develop
in a satisfactory manner.

Fourth stage of control is Pranayaam
Breath-control practised regularly, develops intelligence and the power of concentration. The control of mind is related to control of respiration. Pranayaam is a system of bio-energy control thorough short-quick and long-slow breathing and breath suspension exercises.

Fifth stage of Control is Pratiyahaar (The sensory control)
The control of the mind in respect of sensory objects is Pratiyahaar.

6th, 7th and 8th Stages of control Dhaarna, Dhyaan and Samadhi
Dhaarna, Dhyaan and Samadhi are exercised in relation to the mind. In other words, the control of the mind is practised in three main steps, and each one has a technical name denoting the nature of the control, as mentioned below.

Dhaarna is the 6th stage of control and the first phase of mental control or concentration. Dhaarna means holding an object in your consciousness without it’s transformation into another form. For example holding an image of an apple in your mind say for 12 seconds without letting it slip from you consciousness that is Dhaarna.

Dhyaan is the 7th stage of control and the second phase of mental control or concentration. Concentration is the holding of an image in consciousness continuously and without interruptions by the penetration of any other images, when this concentration becomes very deep by an uninterrupted and continuous holding of an image in consciousness for a sufficiently long time, it is called Dhyaan.

Samadhi is the 8th or final stage of control (super-concentration). To maintain in the consciousness an image of an object without letting it slip or disappear from the consciousness is holding. It is the specific function of the concentrative mind to hold an object in the consciousness without heaving it loosened or escaping from the consciousness. This is called super-consciousness state, the Samadhi state is the final stage of yoga practice.
 
Book title: LAYA YOGA by Shyam Sundar Goswami 1891 – OCT 1978
Ref: ISBN 0-7100-0078-2