AAPL and the SPY are always very highly correlated and AAPL has high institutional holdings which tends to make it a leader of the SPY.
Around the beginning of August there was a divergence in AAPL vs SPY and this led to the SPY following it downward.I think we are experiencing this again.
The trade would be long SPY short AAPL in a relationship to make your risk equal on both sides.
Overall I'm still bearish on SPY but It has many upwards gaps to fill and in 2008, 2011 it did not set new lows after the first drop with any gaps unfilled.
I'm still new to trading spreads but I think this should work out.