Study a single sutra by selecting the pada (chapter) and then the sutra (verse).
Source: English translation from The Yoga Sutras of Patanjali (Sri Swami Satchidananda)
Niyama consists of purity, contentment, acceptance but not causing pain, study of spiritual books and worship of God [self-surrender].
Source: English translation from Light on the Yoga Sutras of Patanjali (B.K.S. Iyengar)
Cleanliness, contentment, religious zeal, self-study and surrender of the self to the supreme Self or God are the niyamas.
Source: English translation from The Heart of Yoga (T.K.V. Desikachar)
Source: English translation of the Yoga Sutras of Patanjali (Swami Jnaneshvara Bharati)
Cleanliness and purity of body and mind (shaucha), an attitude of contentment (santosha), ascesis or training of the senses (tapas), self-study and reflection on sacred words (svadhyaya), and an attitude of letting go into one's source (ishvarapranidhana) are the observances or practices of self-training (niyamas), and are the second rung on the ladder of Yoga.