You will need a stack of fresh material, unknown to you or never played before (libraries and volumes of collections are the best source here, unless you are independently wealthy.)
The material you start with MUST BE COMMENSURATE WITH YOUR CURRENT SIGHT-READING ABILITY, NOT YOUR CURRENT PLAYING ABILITY.
Choose material you can sight read (errors allowed, a fair amount) hands together and at least at 60% of the marked tempo.
Almost everyone has some experience from school or church 'somewhat reading' while singing a single line song or part in a chorus - radically different from two staves and polyphony. The initial habit is to read a single line. Force your eye to sweep not just horizontally but vertically. Literally sit far enough back that your eyes can take in, at least peripherally, one entire line of the grand staff.
You may have to start with the Bartok Microkosmos, one of the most elegant and musical graded sets of pieces yet conceived for learning both piano, sight-reading and rhythm simultaneously. The first piece is in unison, parallel motion. They progress smoothly: initially your hands are over one five-note pattern, then to a piece with a shift of hand position to another five-note pattern. If they are 'beneath' you I would still recommend them as a great starting point. Going through books I - IV, if simple for you, will be a quick exercise, and you will develop, small scale, all the fundamental habits and reflexes you need for more difficult pieces.
VEHEMENTLY REJECT AND / OR ABANDON ANY AND ALL ACRONYMS FOR RECALLING NOTE-NAMES AND THEIR LOCATION ON THE STAFF. Some 'residual' of those God-Damned sentences will forever be attached to your reading, and will, to at least a tiny degree, slow you down, forever, as attached baggage (I'd say downright garbage) which will never completely go away. (In other words, they pull your mind away from the matter at hand, which of course is counter-productive.)
Learn your musical alphabet, reciting every other letter, in pairs, both ascending and descending. This can be done in small increments throughout the day, when, in such mundane situations, as brushing your teeth, etc. After a few weeks of a few moments here, there, several times a day, you should have them near-reflexive (...no more difficult than learning the entire alphabet in threes for library use.) Then you are spelling generic thirds, which are on any staff, a line to line or space to space figuration. Once you have the pairs down, extend that to three letters - spelling basic triads -- then four, spelling basic seventh chords.
It is also well-worth recognizing numeric intervals, by spatial distance, and their lay of the land on the page. Any octave will be a specific distance, with one note on a line and the other on a space: any fifth will be on every other line or every other space. Thirds are space to space or line to line, seconds line-space, etc.
Grab, right now, the coins in your pocket, close your palm on them. (I'll use American currency.) Open your hand, glance at the coins for one second, close your hand. Now name how much money is in your hand...
If you see one nickel, two dimes, a quarter and a penny...
and came up quickly with fifty-one cents, you have just exercised qualitative perception. (coins as icons representing
If your process was
5+10+10+25+1 you are in the habit of quantitative reasoning, the quantity of each individual coin added up in a string.
Most people, unconsciously, use the qualitative mode, because they are familiar with the objects and it is more direct and immediate. It is the qualitative mode you want to develop reading music, where an interval, eventually without thinking, just 'pops' off the page at you, and your motor reflex responds. In brief, pattern recognition. (at that point you are not consciously 'naming notes!')
You want to begin to think of the notes on the staff in that same quantitative manner: Following the directive to 'just learn' the notes in letter pairs, you will have sight-anchors for just about any note or notes on either staff (eventually). The most obvious is to first 'just memorize' what note the middle line of each staff is, then you are literally 'centered' as far as eyeballing anything else in the measure, as relative to that middle line -- you are seeing patterns to which you have developed a motor sense instant reaction, and are no longer 'naming notes' or figuring out 'that is a fifth, and then getting your 'hands around that.'
THERE IS ABSOLUTELY NO USE GOING OVER A PIECE TWICE. Think about it: YOU CAN ONLY "SIGHT-READ" ANY PIECE ONCE! After that, some data is already stored in the brain, and you are not facing the challenge of reading something 'for the first time.' In that stack of sight-reading material, keep bookmarks so you know where you left off, to avoid 're-reading' anything.
Four-part chorales are good. A stretch would to be to read from the four-stave actual choral score, making the octave displacement of the tenor, written in treble clef (Any 'c' part, and its octave re-location Is Not A Transposition - a minor technicality many get wrong.) Now, four-part chorales may be used for going over repeatedly, but with this additional 'drill.' Play just any two of the parts in isolation, the soprano and bass, the soprano and tenor, etc. Once sight-read initially as a whole, this is an excellent exercise. I recommend putting it all back together before moving on to another piece.
None of this will in any way be 'comfortable.' You are pushing yourself in an unpracticed and unfamiliar mental capacity: this is completely apart from your playing ability and usually quite far beyond normal comfort zones. Initially, you will find mental fatigue setting in quite quickly - exactly as you would an unused muscle you were beginning to exercise. It can be mentally exhausting. The moment your brain starts to fatigue, that particular sight-reading session Is Over! You can go back and do another few minutes after some stretch of 'normal' practice.
Looking the piece over before you play is a vitally important thing to do, regardless of how much you currently see or hear, it is precisely the same as looking over a map before you set out on a trip - you at least have a vague idea of the terrain you will be traversing, key, configurations, phrasing, dynamics and cadences. (Regardless of pyrotechnical ability, a player without an idea of the piece, as a whole, and 'what it is about' is not going to give anything near an interesting performance.)
When it comes to configurations, like an arpeggiation, do try and visually 'compress' the horizontal data into one vertical - you will often begin then to recognize a very standard chord or its inversion, and that 'grabs' an entire grouping vs. reading note to note. [ADD: Some classical music, the left hand accompaniment, either arpeggio or Alberti bass, can be read through pushing to play each quarter note as a vertical chord - that exercise is exactly 'compressing' the horizontal visual data into vertical reading.]
The above is a nutshell of what can be a long process, and I can only advocate that you begin, then stay with it. This is a process, and most find that after some time, one day, over your shoulder and in retrospect, it has had its effect. (Ear-training and solfege, for many, 'happens' in the same manner.)
You are not at all alone, especially among keyboard players, of having your sight-reading lag behind your technical playing ability. The self-taught, especially, often find themselves in this situation.