To Thicken a Mesh to Volume

Select the Thicken Mesh to Volume tool ( Modeling > Mesh > Create > Extrude or Thicken Mesh > Thicken Mesh to Volume ).
Select the mesh element to thicken.
Do one of the following:

If Thickness is off, select the thickness and direction with AccuDraw.

or
If Thickness is on, the direction of the thickness depends on whether the Thickness value is positive or negative.
Click to give mesh element volume.

Thicken Mesh to Volume, Thickness = off.