Shorting when the overall (stock) market is in an uptrend is most certainly not the best idea... unless you are going for a hedge against a basket of long positions. You would be looking for the "odds" to be in your favour, a risk reward ratio so compelling that it makes sense for trading purposes. For example $YY here below:
After a strong uptrend, a weekly bearish pin, in this case a shooting star in the jargon of Japanese Candlesticks, appears at a major resistance zone. This confluence favours the downside going forward... but do not forget to set a hard stop (!) as sometimes the market doesn't play ball...