Here are several practical quotes from this profound book, in which de Sales details how to use your imagination during prayer.
There is a third [point], which is not necessary to all meditation, called by some the local representation, and by others the interior picture. It is simply kindling a vivid picture of the mystery to be meditated within your imagination, even as though you were actually beholding it.
For instance, if you wish to meditate upon our Lord on His Cross, you will place yourself in imagination on Mount Calvary, as though you saw and heard all that occurred there during the Passion; or you can imagine to yourself all that the Evangelists describe as taking place where you are.
In the same way, when you meditate upon death, bring the circumstances that will attend your own vividly to mind, and so of hell, or any subjects which involve visible, tangible circumstances.
Often this use of the imagination tends to concentrate the mind on the mystery we wish to meditate, and to prevent our thoughts from wandering hither and thither, just as when you shut a bird within a cage, or fasten a hawk by its lures.