Assuming you already know how to model a box without a sloped roof, and assuming you are trying to model this in the IDF editor specifically, and assuming you want a simple slope; here is one approach.
- Find the roof surface, and increase (or decrease) the z value on the edge or the roof you want to change
- Find the wall surface that sits under the roof edge you chanced and adjust the z value of the upper two points on that wall
- On the two side walls that have an edge both on the low and high side of the roof, change the z value of the upper point on one side to match the wall and roof you just changed.
- The last wall should not need any changes,
