In the Design workspace, click Create > Create Sketch, and use sketch tools to create a 2D board outline.
Click Finish Sketch at the right end of the toolbar.
Click Create > Create PCB > Create Independent PCB and select the sketch.
Alternatively, you can also select the face of an existing solid.
In the Create Independent PCB dialog, optionally set the Origin, specify the X Axis, or Flip the X or Z axis.
Check Convert Circles to NPTHs to create non-plated through holes from sketch circles.
After enabling the check box, set the Max NPTH Diameter value to convert circles that are equal to or less than the specified diameter to NPTHs.
In the Create Independent PCB dialog, click OK. The 3D PCB is created, and the workspace changes to Electronics 3D PCB, where you can link to a 2D PCB, and add additional holes to the board.