Independently or alone

अकेले

English Usage: He prefers to work by himself.

Hindi Usage: वह अकेले काम करना पसंद करता है।

In the company of (indicating a surrounding situation)

के द्वारा

English Usage: The book was written by himself.

Hindi Usage: यह किताब उसके द्वारा लिखी गई थी।

Reflexive form of 'he'

स्वयं

English Usage: He made the decision by himself.

Hindi Usage: उसने स्वयं निर्णय लिया।