First with a part open I am going to create a sketch on the bottom of the part. And i am doing this because i know that i am going to fixture this part and i can define the offset below the part for the amount of material that is going to be in the fixture.
With the new sketch we can sketch out any kind of shape that we need to define for our stock. Here i am going to do something like what is in the pic below.
Next let's pick on the job setup button to go ahead and start setting this part up for programming.
In the Setup go to the Stock section and pick on the pulldown that shows relative Size box.
Now, there are 2 offset values that you are going to work with:
Stock offset: This is basically the distance in -Z in which your stock is going to go.
Height: Overall height of the stock solid being represented.
Once you have these values defined then you are ready to start putting toolpaths on the part!