One thing I will tell you is that it will be a humbling experience lol. You have to really start from the beginning. I figured that since I could play a few more advanced pieces that I would be able to start sight reading at least at half that level. I was wrong.So basically:1. Start at the lowest level you can comfortably sight read at tempo (which might be lower than you expect) and work from there.2. Add new concepts one at a time and master them before moving on.3. Don't try to skip ahead in concepts before mastering them; you need all of them.4. Go for quantity over extreme quality (i.e. don't try to polish every piece. Get it to where you can play the notes at tempo with accuracy and then move on to the next one).
I totally agree with you Quantum. Being able to prioritise the most important elements is a key factor when sight reading. Keeping going IS the key to building up speed, even if some notes, or a lot of notes are inaccurate or even missed completely.
Hymnals are a great place to start. Get one and just start plowing through some hymns without stopping.