Breaking a chain is dead simple with the proper tool. Pushing the pin back in is a little bit tougher. Don't push the pin out too far -no further than you need to in order for the chain to come apart. If you push it all the way out it'll be harder getting it back in. It'll want to skew and bind. Also, there is a technique for loosening up a tight link which you might get when you push the pin back in. The chain tool you get may have instructions for this but basically you use the second set of teeth on the tool to help push the pin while pulling at the side of the chain so it isn't too tight. If you are handy this is pretty obvious and self-explanatory. If not, there is a bunch of info out there online. It's not rocket science. If bike mechanics can figure it out any trained monkey can do the same.
Determining the correct number of links you need is the next thing. It'll be obvious with a single-speed but with a deraileur bike it'll be a bit more work to make sure it isn't too long or too short.
Sheldon has a good page for hints on this.