We will now create another box then snap it to the create box you have already made and move it inside the center of the box you have created.
Using the MAIN TOOLBAR select PRIMITIVES then BOX to create a small 12mm cubed box.
Once the new 12mm cubed box is created will use the SNAP TOOL to snap it to your larger existing box.
Select the new smaller 12mm box. Once selected some arrows should appear that you can use to move the box into the center of the larger 25mm box.
When you select the smaller box if it selects both the larger and smaller at the same time you will need to turn off GROUP SNAPPING
Now use the MOVE option and ARROWS to move the smaller box
Make sure to leave the proper room.
BEFORE PROCEEDING SAVE YOUR PROGRESS!
Go to the APPLICATION MENU in the top left corner, then select SAVE and SAVE TO COMPUTER