Note that publications can be validated automatically after import if a setting called “Auto validate” is activated in the background.
To activate this setting contact support@btwentyfour.com.
To start validating a publication, go to "Products" - "Publications".

Click the three dotted icon on the row for that publication.

Click "Validate" in the pop-up menu.

A pop-up window appears.

Click the checkbox for the buyer.
If validating a publication uploaded to a GLN belonging to a group, all buyers in that group will be listed, and will be selectable via checkboxes.
The validation will run towards the selected buyer´s validation rules for PRICAT.

Click "Validate".

A notification will show telling the validation has been queued and will begin shortly.

The validation process will start and run in the background.

More information:
Step by step guide from uploading to publishing master data.
How to see that a publication has been validated with errors?
How to see that a publication has been validated with no errors?