We'll start off with the grid shown in the tutorial for 1-point perspective drawing, as shown below.
In 1-point perspective, everything you draw fades back to the centre, vanishing point 1. However, in 2-point perspective everything fades back to the additional points you used to draw your grid - vanishing points 2 & 3. This means that the grid needs to be rotated 45 degrees - everything you draw will be drawn from a 45 degree angle. This new 45 degree grid is drawn by simply adding in radial lines from both vanishing points to each point where the existing grid meets the front of your artboard. Then, add more radial lines from the vanishing points to where your existing grid meets the edge of the artboard. You'll end up with something similar to the following image.
The new grid squares at the front of the drawing are 1 square unit, however you'll notice that the radial lines drawn to meet the edge of the artboard seem to be too far spread out. This isn't your eyes deceiving you, simply that the squares spread across 2 units. Therefore, you must add in more radial lines between the ones you have drawn. Your final 2-point perspective grid should appear as follows.
Just like in 1-point perspective drawing, the heights of objects are worked out using the incremental grid along the front of your artboard, and the central vanishing point. Once you have worked out the height, your 45 degree angled objects must be drawn back to the vanishing points at the edges of your artboard. This is demonstrated below.