How to become a good piano sight reader? It does not happen over night. With practice it may take up to a year to learn piano sight reading.
You need to practice sight reading every day (and make it within your current skill level). Did you read Shakespeare in kindergarten? No!
It probably took you years to get to get to the level of comprehension and skill to read books like Shakespeare. Similarly, do not fool yourself into believing you will magically wake up one morning and be able to play a new piece (near perfection) the first time you play through it.
Piano sight reading is a learned skill that comes with a very heavy price tag called practicing. Just like reading words, it took you a long time to get good at reading. Same with sight reading it takes a lot of practice, practice, and practice.
Pick ten music books (books with hundreds of songs), and play each song (just one time). Remember to choose music which are below your current technical level. Remember to go slowly so that you achieve accuracy in dynamics, notes, and rhythm.
As you achieve accuracy, you may want to increase the speed gradually until you able to reach tempo. When you discover inaccuracies creeping in just slow down until you achieve the accuracy you want.