1. Read your piece to yourself to find big problems.
When editing, let yourself be the first set of eyes. ONLY look at big picture items. Things such as topic/theme consistency, relevancy, clarity, voice, tone, imagery, order, message, and length are often looked at in this stage. If you are writing a narrative, or if you have a person in your story, you'll often consider the person's voice or character consistency throughout.
This is the point where you will often cut sentences or paragraphs, change the order, rewrite entire sections of your piece, or change your wording to make your piece more clear. BIG things are happening.
2. Give your piece to a friend for feedback.