Hi Abdulla,
As u know, each qpsk symbol contains a real part and an imaginary part, so in order to form the real part I need the bits with the odd bits, and to form the imaginary part u need the even bits.
e.g
the qpsk symbols are :
1. 0.707 + 0.707 j
2. -0.707 + 0.707 j
3. -0.707 - 0.707 j
4. 0.707 - 0.707 j
so if bit number 1 is 0 and bit number 2 is 0 then the qpsk symbol will be 0.707 + 0.707 j, while if one of the bits is one u have to change the sign of the 0.707 to be negative.
that's all, I hope u got my point.