If I were guessing I would think they would begin by destroying or electrical grid, since we seem to have no inclination to protect it.
I lived before rural electrification was introduced so I know how to survive with out electricity but 90 to 95 percent of our population would not survive, the collapse of electrification.
I beg to differ.
Huge amounts being spent now to prevent cyber attacks on the electrical grid, and other strategic systems as well, such as Gas.
Physical attacks are harder to defend against, but conversely will require an extensive special forces/sabotage unit deployment in the US. This is true of any country though, nobody has an electrical grid hardened against that.