Execution Flow

Setup the Attack: Setup a session: The attacker has to setup a trap session that provides a valid session identifier, or select an arbitrary identifier, depending on the mechanism employed by the application. A trap session is a dummy session established with the application by the attacker and is used solely for the purpose of obtaining valid session identifiers. The attacker may also be required to periodically refresh the trap session in order to obtain valid session identifiers.

Techniques
The attacker chooses a predefined identifier that they know.
The attacker creates a trap session for the victim.

Attract a Victim: Fixate the session: The attacker now needs to transfer the session identifier from the trap session to the victim by introducing the session identifier into the victim's browser. This is known as fixating the session. The session identifier can be introduced into the victim's browser by leveraging cross site scripting vulnerability, using META tags or setting HTTP response headers in a variety of ways.

Techniques
Attackers can put links on web sites (such as forums, blogs, or comment forms).
Attackers can establish rogue proxy servers for network protocols that give out the session ID and then redirect the connection to the legitimate service.
Attackers can email attack URLs to potential victims through spam and phishing techniques.

Abuse the Victim's Session: Takeover the fixated session: Once the victim has achieved a higher level of privilege, possibly by logging into the application, the attacker can now take over the session using the fixated session identifier.

Techniques
The attacker loads the predefined session ID into their browser and browses to protected data or functionality.
The attacker loads the predefined session ID into their software and utilizes functionality with the rights of the victim.

Session Fixation