Player: I attempt to yank and disarm the orcs bow with my mage hand cantrip.
DM: Okay let me look up the description of mage hand *leafs through the phb* alright you can try it.
Player: great, what do i roll?
DM: Make an intelligence attack roll as if casting a spell.
Player: hmm a total of 15.
DM: You hit and your mage hand grabs hold of the bow.
Player: great one disarmed orc.
DM:Not quite, the hand has the bow in its grasp now you need to move it away from the orc. Right the mage hand has a lifting capacity of 20lbs, thats a str 2, thats minus 4 plus half your level, lets say moving something away from a grabbed person you have to beat the opposed grab roll by 5 to yank it. Right lets roll.
Player: 20! er minus 4 16?
DM: 5 er plus 4 for the orcs strength, 9, dangnammit, the bow is yanked out of the Orcs hand and lands in the square in front of it.
Player: Alright!
DM: Right the orcs go, he moves forward and picks up his bow and shoots you in the face.
*i do not condone the dming approach above, honest guvnor*