SO the idea is to create a wall of sticky notes. At this event every time someone enters they have to write something inspirational on a sticky note on the wall. And then stick it. Now I want to make a timelapse video for this. But I'm not sure how timelapse works. What time frames should I keep? How often to take the picture, and then I know when I tried making a clouds timelapse my camera (600D Canon) would not focus, even on manual. And therefore wouldn't take the picture. Any ideas?
But I'm not sure how timelapse works.
A time lapse movie is simply a sequence of photos taken over time, usually at regular intervals.
What time frames should I keep?
All of them, unless there are reasons to throw some away. For example, if you continue taking photos throughout a day, all night, and into the next day, you might decide to toss the night ones if you can't see much or very little is happening. That will of course produce a noticeable jump in the movie, much like a cut from one scene to another.
How often to take the picture
That's up to you. What's the target length of the movie, the number of frames per second, and the real period of time covered? For example, if you want to compress 8 hours into 1 minute, you'd calculate:
60 seconds at 24 frames/second = 1440 frames needed 8 hours * 3600 seconds/hour = 28800 seconds to cover 28800 seconds/1440 frames = 20 seconds/frame
So for this example you would set your camera to take 1 frame every 20 seconds. I don't think the 600D has a built-in intervalometer, so you might need to buy one of those.
when I tried making a clouds timelapse my camera (600D Canon) would not focus, even on manual
If you set your camera on manual focus, it should take a photo no matter where you set the focus. If you're having trouble in this respect you may need to ask another question with more information, or just check your owners manual.
For time lapse movies you'll probably want to set your camera on manual focus and leave it at the same setting throughout.