How is the baseline set in Primavera P6?

Use the maintain baseline and assign baseline dialog boxes

refer documentation or watch video tutorial on YouTube 

Primavera P6 Professional allows you to assign up to three user baselines and one project baseline per project. The following article explains these capabilities and how they can be helpful in tracking the overall status of a project and near-term interim variance reports.

Take a look at the following diagram (taken from the Ten Six Primavera P6 E&C training course) that outlines the concept of project and users baselines.

Current Project

This is the current activities in the project schedule and is editable in the activities view.

Project Baseline

This is a static copy of the project, typically created once the final baseline has been agreed by all the stakeholders. This should be under version control and not changed without due process.

User Baseline Primary, Secondary and Tertiary

These are more ad-hoc snapshots of the project that are for near-term variance and comparison reporting.

Baseline Basics

When you create a baseline in Primavera P6 Professional using the Maintain Baselines dialog, you are in fact creating a complete copy of the project in the database: the baseline project just doesn’t appear in the project list because P6 has it tagged as a baseline copy.

Above: a baseline just created using the Maintain Baselines feature of Primavera P6. It is being labeled as Customer Sign-off Baseline.

However, this “invisible” copy of the project is visible as a baseline in the Assign Baselines dialog, and can be assigned as either a Project, Primary, Secondary or Tertiary baseline, depending on what you need.

Above: see how the baseline copy of the project appears and can be assigned as the Project Baseline. Until a baseline is assigned as a Project, Primary, Secondary or Tertiary baseline, the current project is acting as a baseline.

You can create any number of baselines within each Primavera P6 project; there is no limit to the number the tool allows you to create. However, you should avoid creating excessive baselines as each one is effectively a full project and is occupying roughly the same space in the database as the project schedule. Too many unnecessary copies of projects within the P6 database will eventually affect performance.

First of all a schedule is prepared. 

• Develop the WBS for your project,
• Add various activities, giving them appropriate details like duration, activity type, calendar, percent complete type, assigned resources, etc.
• Link all activities in a logical manner by assigning relationships.

Once your schedule conforms to your requirements in all respects you are ready to set it as the baseline.  

The previous answers are right, in additional to when you doing Maintain Baselinem choose "Save a Copy of the current schedule".

Also when you Assign Baseline, click on Baseline project & on Primary Baseline and select the baseline schedule for both cells, you can also assign more than one baseline.

For the advantages usage, view the Planned dates by Selecting "Columns" then choose BL Start BL Finish, Schedule % Comp., BL Cost or Variance Cost/time..

To view the Baseline Bar chart, Open View, then Bars, Choose Primary baseline

1- After you done your schedule

2-Go to Project Tap Select Maintain Baseline Then add 

3- GO to  Project Tap Select Assign Baseline and select the desired baseline

 Thank You