Advice on short selling SPY or VFV

Hi, it looks like S&P are on a downward trend and the FED rate is not expected to decline. I'm planning to start short selling. Can someone with short sell experience share how to do it and what to watch out for along the way? Thanks for all the advice. $SPDR S&P 500 ETF (SPY.US)$ $VANGUARD S&P 500 INDEX ETF TR UNIT (VFV.CA)$