I would recommend you to read theory of segmentation here, before you read about how to create segmentation within GA. Once you have understood the importance of segmentation and use of it then it would make much more sense about how to create, and implement segmentation within GA.
Once you login to you account, and look into reports, at the top side of the page you would be able to see something closer to the image above. It shows you two blocks, All Sessions and Add a Segment. All sessions means that all the sessions that have been recorded on your digital platform has been included into the analysis within your reports,
As a first step to create and include segments, click on the box which says add a segment. Once you click on that, you would be able to able to view all the segments, the ones that have been created by you, and the ones that have been provided by GA by default.
Some of the default segments are:
1) New Users
2) Return Users
3) Bounced Sessions (know more about bounces from here)
4) Converts
5) Direct Traffic
6) Purchase sessions
7) Mobile Tablets
There are many others, which I would recommend you all to view and test by yourself.
To create a new segment of your choice, click on new segment as in the image below:
Once you click on new segment, it gives you an option to create segments on following parameters:
1) Technology: On basis of device type, resolution, browser/OS type, model etc.
2) Behavior: How many sessions, days since last session, session duration etc
3) Demographics: Age, gender, language, location etc
4) Traffic sources
5) Date of first session
6) E-commerce related: Revenue, date of purchase, transactions etc
7) On the basis of custom variables
8) Even the sessions which have follow a particular sequence of steps
Once you have put down the conditions of these segments then you can name them and save them by clicking on the save button as in the image below:
Now you can go back to all segments and check on the segments that you would want to analyze and click on Apply, as shown below:
As an example you can view the report below, where I have activated 2 segments, based on traffic sources, email or facebook:
