Sight-reading does not require a single skill but rather a number of skills perfected together. There needs to be mastery of the keyboard, mastery of musical notes and understanding of the musical score. The first step is to review the score thoroughly.
The left and right hands will be most likely playing very different things, so make sure you give the score a good look over before beginning.
You also need to make sure that the piano is correctly tuned and all keys are functioning well. Check that you are seated in a comfortable posture with the score placed in line with the front eye view to avoid straining. Students who have mastered the keys do not focus on looking at the fingers when playing, but those who have not mastered the keyboard need to find a balance between reading the score and looking at the keyboard.