When does a knitter begin the actually knitting process of a specific work such as a sweater or mittens? Scriptures tell us that God knits you together and science tells us that the process of the specific individual human being starts within minutes of fertilization. Even the earliest structures 'knitted' for the embryo (the yolk sac; the layer of the placenta which forms to the umbilicus and placental barrier, etc.) are incorporated into the later structures of the fetus (as gut and connective tissues). The form and function of the newly conceived individual human life at every age following fertilization are the hallmarks of that individual's body and personhood.
Since I cannot point to a specific moment during the lifetime of the individual begun at fertilization as the advent of the soul, I would defer to the logical position of assuming the knitting process of body and soul (soul to body) begins with first cell division that evidences the newly conceived expressing its life and its effort for survival of the individual at earliest age in the lifetime.
[One more aside, though it may too esoteric for this discussion: it is assumed that the soul is a timeless 'thing' since it continues even after the time-sequestered body decays and returns to constituent atoms, thus the notion of a soul existing before the body is not inconsistent with the knitting process whereby body and soul are joined ... when God creates that soul is and will remain in His purview, but don't assume that the miscarried bodies did not/do not have a soul in God's timing or His creative endeavors. As a Christian, I believe that your soul will exist sans body, but that you will again inhabit a body of God's special creation at some where/when of His choosing. It is entirely within the realm of acceptable belief for me or you to allow God the creative power to produce a body of completeness for even that miscarried embryo/zygote, not unlike the body you will inhabit in some where/when of His choosing, perhaps long after your natural death and dissolution of your current body.]
When he conceives the idea.
Helps explain how God knew us before we were formed, even.
Regards, Marvin.
See post #268, and add Substance and Accident to the list. The reading may seem arcane at first, but it's actually a fairly straightforward, rigorous application of common sense.