Edit STL file and perform a very simple cut My goal is simple and as I have seen, there are multiple methods to perform a mesh "cut" but most are examples using a simple cube or sphere object.

Being brand new to 3d prints, there is an object on thingverse that allows for customization of the design.  I want to remix and improve this design by adding a simple opening "cut" in the strap buckles that is commonplace to allow strap insertion at an angle without having to rethread the entire thing (in addition to what the online customizer also allows me to adjust).  Pretty simple stuff. Until I launched blender. :)

I want to cut about a 2-3mm diagonal line through the outer buckle on each side.  As I understand it, the ways to do this are try to create cube objects, somehow transform it after rotating and moving (which I still cannot even figure out how to do easily) then using that object as a delete "template" to carve out that shape from my target.  Or, theres the knife tool. Or, there's the knife "project".  Or, there's the boolean way. 

I use Photoshop extensively, I am not an artist but am very adept at photo and video editing etc. but not a CAD guy and I cannot seem to get this thing cut properly.
Well, as you seem to have discovered, there is no shortage of possible ways to go about this.

Without going into too much detail, I would probably use a boolean modifier in this case. To do this:


Import your STL file (File > Import > STL)
Create an object roughly in the shape of the hole you'd like to create in the STL.
To do this: 


Add a cube (&#8679; ShiftA> Mesh > Cube)
Scale it on the X axis to make it thinner (S to scale, X to restrict the scaling solely to the X axis,  LMB or &#9166; Enter to confirm and exit the scale operator)
Grab (G), rotate (R) and scale (S) the cube into position such that it intersects the buckle object where the hole should go.



Add a boolean modifier.
To do this:


Select the buckle object (click on it with  RMB) and add a  boolean modifier in Properties > Modifiers
Set the object added in step 2 as the "Target" object in the boolean modifier
Set the boolean operation to "Difference"
(Optional) Set the cube's Maximum Draw Type to Wire in Properties > Object > Display, so you can see the result of the boolean operation better



Adjust the cube further if needed
Export the result. Either Apply the boolean modifier and delete the cube object before doing this, or enable Selection Only in the export options and ensure that the cube is not selected.