Well first you would need to remove the Crank bolts on the arms, they usually take a 15mm socket.
After you get those off you'd want to use a crank puller. You can check out the Videos, there is a tutorial on how to remove the cranks and one to remove the bottom bracket.
You might not actually have to remove the bottom bracket if your just putting on a new crankset and it uses the same bottom bracket.
Not sure without seeing the crank. Post a pic if you can. The M521 is an Octalink crankset, similar to the ISIS standard. Some cranks are of the self extracting kind (Like my Truvativs) and don't require a crank tool. Watch the video at this site, and go from there.
Dedicated scholar of bicycles
See Park Tool:-
You could put a thin flat metal disc inside the crank end which is just big enough to rest on the end of the tube. (or use a washer where the center hole doesn't allow the tool end to go through?)
All the Park tool is a standard crank puller with a "plug" on the end.
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