After introducing yourself to the Illustrator perspective grid, it's time to put it to work!
Choose an image; this image can be a drawing or a photograph. Once you open this image in Illustrator, click on your layer and choose 'template' to convert this image, which will serve as a backdrop guide for your perspective drawing, into a template. This will also automatically set its opacity to 50%.
Once you've placed your image, choose the perspective tool and you will immediately see the grid with its depth and distance mapped against your image. It is easy to see immediately how you can begin to apply dimension to your image of choice.
You will also see a cubical shaped control widget in the upper left of your open file and as you pass your mouse across the three faces of this widget, it indicates which grid you will be applying your image to. For starters, with the default left grid selected, begin drawing a rectangle. You will see that it is applied to the angular grid you are mapping to. Notice that the upper left face of the control widget is now blue. Click the right side of the widget to apply a shape to the right grid and see how the widget highlights this right face with orange. This is Illustrator's simple way of having a visual reminder of which grid you are working with, using a highlight color that matches the color of the grid itself. When you have a grid selected, even if you draw outside the area of the grid, you will still be drawing on that perspective, at that same angle.
These same choices can be made using the numbers keys, 1-4 on your keyboard sequencing through the left, right, ground, or no active plane options. As you begin to experiment with this perspective technique, use the grid tools to seek alignment with your object. Move the horizontal grid to match the 'ground level' of your image. If your image has sloping foundation, use the grid control to match this slope.
You want your grid to match the alignment and slope that exists in the image you are using as a backdrop. It will help to zoom out providing greater visibility as you stretch your grid across the image, visualizing their alignment. If for example, your image view is at a sharp angle, you will need to match this with your grid so you will simply need more screen 'space' to drag it out enough to view and match. Once you are comfortable with your grid overlay, you may want to dim it a bit using the View->Perspective Grid->Define Grid menu option which presents several viewing options including opacity. This option also displays the exact angle and grid density you defined, applying visually using your mouse.
When I use both precision and graphic tools it is useful to compare the results and use them to refine one another. If you visually apply a plane, then see that is it a 51 degree angle, you can use this information to match exactly or create geometric contrasts at precise angle blending the 'looks' with the 'real numbers'.
Once you are comfortable with the relationships and tools to manage them, begin with a simple shape, something to accent your image on the same plane. This is a great time to introduce yourself to the other perspective drawing tool, the 'perspective selection tool' that is a 'click option' on the same tool.