For this example I've created a cube primitive
and deleted two front faces which will be recreated for 'Create New
Faces' example.
*Using Front view
Select mesh and go to Edit Mesh Dialog.
Press Vertex if creating new vertices or Face for faces.
Now Create button is enabled.Press it.
A) Vertex:
Start creating new points by clicking in a viewport (dont use perspective).
Vertices depth value will be set to 0 so you must position them manually.
i.e if you create a vertex in Front view (x,z axes for screen x,y) y will be 0.
In this example only one is created.Will help build a face in the next example.
B) Face:
Click on three vertices for each face.Clockwise order.
First I create the cube's missing faces and then an extra face(3) using new vertex.
Press Create button when done.
Final Cube:
If you want to create a mesh from scratch just create a primitive(i.e plane) and start creating vertixes,faces.
Delete its original vertices when you create your model.