Patanjali's Raja Yoga is most simplest to practice among all the ancient Yoga systems.
System of Raja yoga according to Patanjali.
Raja yoga or Ashtanga yoga consists of 8 limbs
1) Yama - Consists of (Ahimsa (non-violence), Satya (truthfulness), Asteya (non-stealing), Brahmacharya (celibacy), and Aparigraha (non-covetousness).
2) Niyama - Consists of Shaucha (internal and external purity), Santosha (contentment), Tapas (austerity), Svadhyaya (study of religious books and repetitions of mantras), and ishvarapranidhana (self-surrender to god, and his worship),
4) Pranayama - Pranayama checks the outgoing tendencies of the mind. It is often misunderstood for breathing exercises. Prana means life force, while yama means to gain control here pranayama involves the regulation of prana or the life force.
5) Pratyahara - Will-power, pratyahara involves regulating the sensory organs so that the energy is not diverted to the sense objects.
6) Dharana - Concentration
7) Dhyana - Meditation
8) Samadhi - Nirvana
By practising yama and niyama, the Yogic student purifies his mind.
By practising asana, he gets steadiness and firmness of body.
By practising pranayama, he removes the tossing of mind and destroys rajas and tamas.
By practising pratyahara he gets mental strength peace of mind and inner life.
By the practice of dharana, he gets ekagrata state of mind.
By practising dhyana, he fills the mind with divine thoughts.
By practising samadhi, he destroys the seeds of births and deaths, and gets immortality, the final beatitude.
All of these steps, are progressions in levels of Raja Yoga and there possibly is not a way of skipping any of these.
Sahaj Marg / Heartfulness mediation prescribes Dhyana / Heart meditation and considers itself a derivative of ultra shortened Raja Yoga.
Sri Sri Ravi Shankar's basic course of Art of Living is heavily focused on the Pranayama, while laying emphasis on Yoga Asana. However AOL is more of a derivative of Hatha Yoga, but if one practices without guidance and supervision can lead to health complications.