The biggest mistake we make when starting to sightread is that we don’t have a plan. A method which gives us a step by step list is best.
For example, decide whether you are reading by interval (note to note), or by key and scale placement (where the notes land in the scale pattern. Also determine the key, whether it’s major or minor and find your starting pitch.
Look for trouble spots. Go through the piece and see if there are large interval jumps, key changes, repeat signs, etc….. Make notes and notations. This will give you more info as you read along.
Come up with your own checklist to use every time you have to sightread something. Then you have a plan and you are not just jumping in with no references.