Open Hand damage is bonus striker damage, like Sneak Attack or Hunter's Quarry. You roll the extra dice when you hit if circumstances allow - which in this case would be whenever the monk has combat advantage for a reason other than flanking, or if the monk is making an opportunity attacks, an attack that he's readied, or an attack against a target that is marked (by anyone).
So a level 1 monk weapon master striking with a dagger using Martial Arts Strike would do 1d4 + Dex modifier when the circumstances didn't apply. When they did, he'd roll another 1d6 (or 1d8 if he has the Vicious Open Hand feat) and add that to the basic 1d4 + Dex mod weapon damage. Note that a critical hit maximizes the basic damage but not the striker damage (you'd still roll the d6, but not the d4).
You might be able to adapt the
Gen Con tournament pregenerated characters to save you some time - there are two level 4 monks in there, one kick-boxer and one grappler. Customizing one of them as a weapon master shouldn't be hard to do.
