The 3D Cursor is a point in 3D space which can be used for a number of purposes.
There are a few methods to position the 3D cursor.
Direct Placement with the Mouse
With the cursor tool enabled, using LMB in the 3D Viewport will place the 3D cursor directly under your mouse pointer.
The view space is used to control the rotation of the 3D Cursor.
For accuracy you should use two perpendicular orthogonal 3D Viewports, i.e. any combination of top Numpad7, front Numpad1 and side Numpad3. That way you can control the positioning along two axes in one view and determine depth in the second view.
By default the depth of the geometry under the cursor is used, this can be disabled using the Cursor Surface Project toggle in the Preferences.
The Snap Menu which allows the cursor placement relative to scene objects.
3D Cursor Panel
The 3D cursor can also be positioned and oriented by editing these values:
The location of the 3D Cursor.
The rotation of the 3D Cursor.
- Rotation Mode
The Rotation mode of the 3D Cursor.