how to design the scale space defined by lowe in sift?

hi everyone, i am trying to implement the sift algorithm mentioned by lowe. but in that i have a problem with how to apply gaussian to the 2nd octave. with what value of sigma we have to apply gaussian. In accurate keypoint localization step, can we add the sigma change to the previous sigma like we are adding for x,y.? and if the offset is a large value(e.g 5) still we have to add it to x and y?