You are very likely going to need a vise.
If you have one, line it with a coupla shims of relatively hard wood. Some folks like leather. Just as good.
Clamp the bolt in the vise but only clamp the part of the bolt that is to the left in your photos. You want to avoid squeezing the legs because, depending upon the material you use to line the vise, you could either sweeze the legs closer together or at worst you could break one of the legs.
You want to get an idea how much has to come off of the bolt before you put it into the vise. Do this by loose fitting the bolt into the cylinder notch as is shown in the Pettifogger article number 2.
Then with one of those fancy files you bought, go to town CAREFULLY on the bolt narrowing it down until it fits in the notches.
My reading of history convinces me that most bad government results from too much government. Thomas Jefferson