My neighbor made herself a sidewalk using that same form. Just please be sure to rotate it a quarter turn every time to keep the random stone look. Otherwise you wind up with a pattern that screams "Did it myself and didn't read the instructions". We have black clay here and after eight years her walkway is still doing great. All she did was remove the grass and level and tamp the path.
If I remember rightly it takes one bag of Quikrete per form. That's a lot of heavy sacks. If you buy them all at once the store will probably deliver them but they have to be kept absolutely dry or you'll get a lot of bag shaped concrete blocks. Even high humidity will do it over time.
You could also buy a few every weekend and within a few months have it all done.
About the only thing that I don't like about my neighbor's walkway is that the stones are a little too rounded on the edges. That makes the lines between the stones too wide. It isn't the most pleasant thing to walk on as I'm always worried about tripping even though it's level. You could fill in the gaps with sand or just set the form a little deeper and allow the gaps to fill in.
There is a mixing barrel that is supposed to be for this.
http://www.hsn.com/products/odjob-concrete-mixing-barrel/5457884Reviews are mixed. My neighbor just used a low sided tub for mixing hers. It worked great.
Good luck with your project.