I'm not an expert on the matter of layering when it comes to electronics and such (for that check out mark2222's saves), but I think I know enough to be able to help. Be warned - there is a little computer-speak!

Simply put, you use the console or the PROP tool to edit particles on top of each other.

You use the prop tool or the console's "!set" command to modify the x & y coordinates of a particle - set two particles to have the same coordinates, and boom, you have layering.

To achieve basic layering:

Draw 2-5 (solid) particles side-by-side. Open the console (~) and type: !set x all # (where # is some number, 300 for instance). This will move all of the particles so they sit at x coordinate 300. [Press d to show the coordinates of the cursor]. If the particles were lined up, they should be on top of one another at x-300. The particles can be whatever you want, but know that non-solids tend to fall apart and volatile elements stay volatile - if you layered TTAN and BOMB, it'd explode instantly. Also, TPT has a 5 layer limit - any more and it collapses into 1 layer thick BHOL.

There are a whole bunch of exceptions and special rules I've skipped (FILT does weird things), but that should be enough to get you started into layering. I'd be happy to explain further, but didn't want to type an encyclopedia's-worth all at once. If you want me to make a tutorial save - let me know, and I'll get on it!