If a previous spackling job left you with rough edges or an unblended texture, you can remedy this by using a drywall sander. A hand held sander is a grip-able applicator that you can set sand paper on for smoothing out surfaces. You will want to use 100-grit sandpaper to level out really rough, high-level spots and use 200-grit sandpaper (which has a more even, gentle distribution of grit) to smooth out the texture.
If you have any low or deteriorated spackling spots from your previous application, you will need to fill them in with a new application of spackling to maintain an even level with the areas you just sanded. To prevent a poor spackling job in this new application, apply a thinner coat and go over it with big, damp sponge to even it out.