It's a great creative exercise to sit down and simply describe what you observe. However, when writing a descriptive essay, you often have a particular reason for writing your description. Getting in touch with this reason can help you focus your description and imbue your language with a particular perspective or emotion.
Example: Imagine that you want to write a descriptive essay about your grandfather. You've chosen to write about your grandfather's physical appearance and the way that he interacts with people. However, rather than providing a general description of these aspects, you want to convey your admiration for his strength and kindness. This is your reason for writing the descriptive essay. To achieve this, you might focus one of your paragraphs on describing the roughness of his hands, roughness resulting from the labor of his work throughout his life, but you might also describe how he would hold your hands so gently with his rough hands when having a conversation with you or when taking a walk.