Sight reading can be split into two separate entities - timing and pitch.
They are unrelated until put together to make tunes. Start with tapping rhythms from the dots, counting all the time. Eventually, you'll get to recognise some common patterns - just like you do with groups of letters that make words.
When, on a tune, you have established its rhythmic shape, then play the notes written, in that rhythm. Start with simple tunes that don't stretch too much, particularly on piano, so your hand doesn't have to float all over, but is stable. One hand at a time initially.
Look ahead, just like you have to when reading text out loud. Sometimes a metronome can help.
Get some sight reading material - ABRSM produces books for this purpose - the grades keep you on track.
Guitar is somewhat different, with so many different places to play the exact same notes. Learn some scale patterns, and adhere to them while sight reading.
Learn intervals, especially using the key root as a start point, rather than plodding through thinking 'C-E, M3, E-G, m3, G-A, M2. Instead, M3, P5, M6.
Sight singing, best done at the piano. Sing a couple of notes, after giving yourself the start note (obviously!) then play them, to check.
It's a long process, and needs doing very regularly, but it obviously worked when you began to read words, didn't it?! Playing with others is another opportunity to get better, once you're happy reading at more than the beginner stage. Good luck!