Step 1: Create a convincing reason to leave. "Sorry, I have to go, I left my cat on the stove."
Step 2: Make a dramatic exit. Slam the door, toss a glass, or play a power ballad.
Step 3: Don't look back. Keep your eyes on the horizon, your feet on the sidewalk, and your phone off.
And that's it! You've successfully crashed the party.
For more advanced techniques, see Advanced Techniques