The minstrel's manual can be acquired by becoming level 40 and talking to Tom Foolery. (Quest# 102 Prats the Way to Do It!) He'll ask you to perform a Critical Pratfall on two monsters. There's really no special way to do it, it happens randomly. But you can speed things up by having some of your other party members learn Pratfall as well.

You'll know it happens when it says, "Critical Pratfall!".

Minstrel's Manual can increase the attack of your whole party by a little. Anyone can use it, not just a minstrel.