How to achieve Camera Facing Billboards?

In most recent books there is a heavy use of camera facing billboards to create the grass layer where the positioning is achieved on the GPU (Shader).

I have some basic knowledge of HLSL Shader and their implementation in 3d Gamestudio. However i do not have any clue how to build camera facing billboards.

Is it possible to create this without jumping into DirectX programming.

The issue i completely do not understand is how to manipulate the vertices because in the vertex shaders i use so far i have only access to one vertex.

Thanks in advance