Sheesh, guys, there's a lot of hate for cottered cranks here! Let me make a couple general points.
If you hate cotters and want to rid the world of cottered cranks, you are not the right person to answer the question of how to service a cottered crank.
If a crank hasn't been overhauled in many years, you are likely to have trouble. But this is a general rule, not unique to junky cottered cranks.
Junky bikes tend to have junky components, and for the last 30-40 years cottered cranks have been relegated to junky bikes. Not surprisingly, a lot of them are junky. Junky components are hard to work with. Again, this is a general rule, not unique to junky cottered cranks.
Sometimes, it all goes wrong, even if you use the right tools and the right techniques. You can minimize the chances of this by using the right tools, or you can maximize the chances of this by using the wrong tools. Again, a general rule.
Okay, all that said, If you actually own a bike with a cottered crank, and you intend to maintain it properly, you should know how to do it.
First step, put away the hammer. Yes, you can remove cotters with a hammer, but it's not the best tool for the job. You can also remove a square taper crank with a hammer, but it's not the best tool for the job.
The bench vise and socket method is a good way to go if you don't have a cotter press. A
drill press vise is easier to handle and is strong enough for the job. A
ball joint separator also works pretty well. A lot of guys have made an excellent cotter press from a cheap
chain breaker, but that requires a bit of work with a drill and a file.