First off you need to set up the spell in the Data base
Set it up something like this
Now enter the Monsters party section of the Database, And Set up a battle event with the condition of the switch Steal Being on. (for now we'll assume you named the switch steal)
Then Set up a Fork Conditions With the condition being a switch called already stolen being off. (make sure you have an else case.)
It should look something like this
In The first part of The Fork Conditions, (where the Black arrow is pointing )Set up a change variable command. Set it to Rand and set the range to 0 to 10 (or whatever type of odds you wish) the Variable set up should look like this.
In the else Section put a message, "you already stole"
Then Set up ANOTHER Fork Conditions. So that if the Variable Stealing(the Variable you just changed) is Smaller then 3 (or whatever Value you wish) Then You receive an item. (make sure you include a message as to what item it is) Have the else statement with a failure message. And Turn on the Already stolen switch.
It should look like this
At the end of all these Commands, Change the switch Steal off.
Now for the final Step. Go to the Common Events Page in the database Make a common Event whose Condition is that the Switch already Stolen is On, in the event just turn the siwtch off
It should look like this
This makes it so you can steal in future Battles.
Now for Some theory, If you set this up the way I've shown you. You can Only steal ONCE in a battle. Perhaps through the use of Variables, you can set it up so you can steal from as many monsters there are in the fight.. (Eg. if there are three monsters, you can steal three times)