What you need to begin with is a picture, once you open your picture onto Photoshop you will need to take its outline like in the example below.

Next you need to start working on the colouring. Its best to choose the colour of the skin as it is a solid colour to begin with.

The way you work with the colours is one layer and one colour at a time, you can see at the picture below how the colours are applied into layers, with the skin colour being the basis for the rest that will follow.

Once you finish off with the colours the most important part of the process begins that gives your picture the cutting edge, which is shading.

Again the skin is the part you want to start first as it is rather easy to shadow and highlight and works as a basis for the shadows and highlights that will follow.

After you are done with the skin effects the clothing is what you want to focus on as it is more painstaking as you want to give it a natural flow and foldings. Once done its time for the accessories and other details of your image and lastly for the hair.

The hair is in reality the most time consuming part of using the vexel process as it takes several strands and highlights to represent correctly reflected light. Finally give your image a matching background that will compliment it and you are done!