I'm another sight reading impaired learner, although I'm not all that awful at it.
Key signatures seem to be the "key" to learning to sight read. At least it seems that way to me. I can sight read the easier keys (CM, GM, etc) at a reasonable tempo for learning/playing/picking out the notes, but the more "advanced" keys are harder. It's the same notes on the same staff, but for some reason they're harder to read fluently. I don't know why, maybe that's just me.
Even with that I still have to write in sharps/flats/naturals on the score or I miss them when playing. I sometimes miss them anyway and it takes several days/weeks/months before I realize it and have to go back and correct myself.
One thing I know is that practicing at the Sight Reading Factory (NOT a plug) has helped in reinforcing what I know and I'm pushing the boundaries of where I'm weak. It's slow, but I am improving. Read the score, hum or sing the score, play the score, play to tempo, then play along with the MIDI as it plays the score to reinforce the lesson. Each one you do shouldn't take more than a couple of minutes to complete.
Another thing: I used to scoff at it, but I now think the RCM requirement of sight reading is invaluable if you're going to learn higher skill pieces. If you can't read the score fluently, then you're not going to put the music together as well. This takes years as you go from beginner to advanced so I'd expect it to take awhile even if you're an advanced player but a horrible sight reader. Older beginners will probably take a bit longer but, as we all know, it's not impossible to learn how to play better.
In the end, the only thing I can say is to start with easy stuff. Any score will work. The steps are: Speak the notes one by one, speak and play the notes one by one, put it together to play the notes, play fluently at tempo, and work upward and outward from there. It only takes 1 or 2 lines of sheet music (8 measures) and you should be working in keys signatures you're readily familiar with to start and then go on to the rest.
For Cannon and a time crunch, I'd make sure I had a bunch of easy pieces in DM and practice sight reading with them before every session working on the performance piece. 10 minutes sight reading daily until you don't need it to read Cannon at tempo or are playing strictly from memory.