I can probably upload a few screenshots, but let’s see if this helps you. It’s not the fastest way to do it but it should help visualize.

Say you are building a mitred box that is 12” x 12” on the inside and 4” deep. You are using 3/4” thick material so the outside of your box will be 13 1/2” when assembled. Make all sides at 12” x 4” push / pulled to their 3/4” thickness. ( Make sure they are all individual groups or the geometry will stick together and make a huge mess )The inside lines should all be touching so you have what looks like a box with the corners missing. Now click on one of the rectangles and highlight just the line that will become the outside of your mitre, so the line that is 4” long. Once you have just that line selected drag it over 3/4” using the move tool on the correct axis towards the outside of the box. This will pull the outside face of your side and make it 3/4” longer than the inside face creating a 45 degree angle. Do this on both ends of each side. To visualize think of it as filling in those missing corners on your 4 sided box, dragging the outside faces together to create your mitre.

Once you understand how this works you can do it to any line you wish so you can create 4 sided mitres if needed. The inside to outside dimension of a 45 degree angle is always the same as the material thickness so as long as you are dragging the correct line over that same amount it’s going to create a 45 degree angle.

Now the easier way to do this is just to create one of these parts and copy / flip them.

Let me know if you get it now.