No one can pinpoint the bottom....so if you think we are near the bottom, start SIP now and after every x points down buy another lot in SIP.....if you feel that the market can go down more, then also start SIP now and increase the value of x or the the points difference below which you will add SIP installments.....
In long term it makes no difference where you buy...what makes the difference is how many years you held on to it......
Smart_trade