Three methods, in order of ease (easiest first):
1) Var 30 tool, with 30/2 attatchment for Stronglight cups.
2) Sheldon Brown method, post #4. When doing this, you need to turn the bolt that is actually inside the frame on a PX (french thread). A nice, deep socket is key here; pick one up when you buy the bolts. As Kurto says, serious torque is needed. Cheap, you can get the parts including the socket for less than ten bucks.
3) Bench vise. IME, least likely to work and most frustrating.
Try DD's suggestion and the others as well; it's best to eliminate some easy stuff before you try the hard...
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