In the bottom left corner of 3dviewport there is a white box, select it and change to node editor.
In the Node Editor header you will notice next to the Node menu are 3 buttons that when hovered over show Shader Nodes, Compositing Nodes, Texture Nodes. Shader Nodes requires a material and Texture nodes requires a texture.
For this part of the wiki document,
Here is Blender Node example. It fixes the normal colors:
Select the Node Tree Type: Compositing Node and click the checkbox "Use Nodes".
Under Add menu located in the Node Editor header you will see the Search menu item. Click it and enter the word Image or browse the menu items to find Image.
Select Image and it will attach a Image node to your cursor.
Place it where you want and then load the image into the node using either the open button located in the node or using the browse button located next to it.
Link nodes by dragging from one nodes output socket to another nodes input socket.
Repeat these steps for any other nodes you want to add and link.