This assumes you have a level to put the cube in. If you do not, the basics of designing level are taught here. First, make sure you have downloaded files, which are needed for creation. If you do not have these files installed, you can continue using a normal cube, but it may mislead the player into believing that it is only a normal cube.
1. Create a prop_physics with the following settings:
Property Name Value Name explosive_cube World Model models\explosivebox\metal_box.mdl
2. Create an env_explosion with these settings:
Property Name Value Name explosive_cube_explosion Parent explosive_cube
3. Place the env_explosion in the center of the cube.
Once that simple process is completed, the cube is ready.
Triggering the Cube
Triggering the cube is very simple. Choose what you want to trigger the cube, and create three outputs. In this example, the input was a button: