This is a very easy project to do, and doesn't require a lot of time- it is up to you how much you want to invest. The Caleveras Clock I did was sort of random, and not planned out ;) ... The glass on my Social Distortion clock broke so I was looking at this plain one for a rescue mission, and next thing I knew I was doodling and pasting on the face of the clock.
Take the battery (s) out. I used a clock w/ a plastic front, so I found 3 plastic tabs on the back of the clock, pushed them in a little and CAREFULLY pryed the cover off. The tabs were at 12, 4, and 8 o'clock, but I'm gonna guess that each clock is probably different.
If you are gonna' do a collage, or glue things on there, you MAY have to remove the hands, which I had to do. If you can avoid removing them, I'd recommend keeping them on there, and just move them CAREFULLY out of the way as you work. Look at how they are sitting, in case you bump them (the hour hand will be lower, then then the minute hand, and then the second hand). [this pic is the best i can do to show the clock hands without taking off the face again & that's definitely not happening ;) ]
Next is the fun part, drawing and/or collage the face. If you did collage, wait for the glue to dry. Make sure none of the mixed media things you glued in there are going to block the movement of the hands. Otherwise, you might as well just turn this into a collage because the clock-part isn't going to work. (If you are just drawing, this part won't apply to you, but don't lean on the hands while you're drawing!)
Now, make sure the hands are in place & on track, and everything looks how you want it, because you are gonna' close your clock face. So carefully pop the cover back on. Put the battery (s) back in, set the time, and now you've got a really cool clock that you made, and all your friends can admire!
**If it's not keeping the right time, change the battery. If it still isn't change the battery again. And then try opening it up and looking at the hands again-they might have been bumped off track as you put the cover over the face.