Just like with any art form, it is difficult to sketch when using poor quality (or the wrong) materials. You can easily find all the proper sketching materials at a local arts and crafts store. Spend a few dollars and gather up all the right materials, including:  H pencils. H pencils are the hardest pencils, and are used for sketching thin, straight, non-blend-able lines. These are mostly used for architecture and business sketches. Get an assortment including 8H, 6H, 4H, and 2H pencils (8H is the hardest, 2H is the softest). B pencils. B pencils are the softest pencils, and are used for making smudged and blurry lines and for shading your sketch. These are the favorites of many artists. Get an assortment including 8B, 6B, 4B, and 2B (8B is the softest, 2B is the hardest). Fine art paper. Sketching on regular printer paper may be easy, but the paper is thin and doesn't hold the pencil as well. Use fine art paper with a bit of texture for the easiest time sketching, and for the best overall appearance. For beginners, it is easiest to sketch from a live model or an image, rather than by using your imagination to create an image to draw. Find an image of something you like, or look for an object or person around you to draw. Take several minutes to study the subject prior to beginning to sketch. Pay attention to these things:  Find the source of light. Locating the primary light source will determine where you sketch the lightest and where you sketch the darkest. Look for any movement. Whether actual movement from a real-life subject or perceived movement in an image, determining movement in your subject will determine the shape/direction you make your sketch strokes. Pay attention to primary shapes. All objects are made up of a combination of the basic shapes (square, circle, triangle, etc.). Look for the shapes that underlay your subject, and sketch these first.

How would you rephrase that briefly using Chinese?
准备好素描用品。 确定素描对象。