Funny you should ask, since I've been leaning how to do this in just the last week.
Rectification is the deriving of an exact time for a nativity whose time of a birth is unknown. One approach to the problem is the astrological analysis of the pre-natal epoch.
The Pre-Natal Epoch is a very ancient concept.
Known as the Trutine of Hermes, or the Trutina Hermetis, and attributed to Hermes Trismegistus, it is cited in the 51st Centiloquy of Ptolemy’s Tetrabiblos, but is now generally considered to have been written in the 10th century by the Arabic astrologer Ja’far Ahmet ibn Yusuf ibn Ibrahim al-daya.
Simply put, the Trutine is the relationship between a normal period of gestation (approximately nine months before birth) and the moment of birth, depending on the Moon and the lunar rhythm.
This sets up a correlation between the chart for the moment of conception and the chart for the moment of first breath, when the physical body separates from the body of the mother.
The Trutina Hermetis method of rectification is based on the assumption that the positions of the Ascendant and the Moon in the conception chart and the birth chart are interconnected by a rule: the Moon at birth is on the horizon axis of the conception horoscope, and the Moon at conception is located on the horizontal axis of the radix.
In classical form the Trutina Hermetis says:
• When the radix Moon is waxing Moon and above the horizon, the period of pregnancy was less than 273 days, and the Moon in the prenatal epoch chart is conjunct the Ascendant of the radix.
• When the radix Moon is waning Moon and above horizon, the pregnancy was more than 273 days, and the prenatal Moon has the same coordinate as the Descendant of the radix.
• When the radix Moon is under the horizon and waxing, pregnancy was more than 273 days, and prenatal Moon incorporates the radix Ascendant.
• When the radix Moon is under the horizon and waning, pregnancy last less than 273 days, and the prenatal Moon conjoins the Descendant of the radix."
http://www.zaytsev.com/EngUG/trutina.html
(If have difficulty with this link then might be easier to do a google search on the title).
In days of Claudius Ptolemy (Tetrabiblos) conception was regarded as the "genesis of human seed" while the birth moment was regarded as the "genesis of man."
"Since the chronological starting-point of human nativities is naturally the very time of conception, but potentially and accidentally the moment of bith, in cases in which the very time of conception is known either by chance or by observation, it is more fitting that we should follow it in determining the special nature of body & soul ... in predictions affecting individual people we have one and many starting points.
"But if they do not know the time of conception, which is usually the case, we must follow the starting point furnished by the moment of birth and give to this our attention, for it too is of great importance and falls short of the former only in this respect - that by the former (the conception) it is possible to have foreknowledge also of events preceding birth.
"For if one should call the one "source" and the other, as it were, "beginning", its importance in time, indeed, is secondary." Ptolemy (Tetrabiblos).
Now either before or after you have done this gather several significant events, hopefully with exact know times attached. Then you can validate. Something will be hitting the angles by solar arc or transit. This has worked with every chart I've checked except mine. My Moon is below the horizon and on the eastern side of the chart at birth. There was a day that fits perfectly with my conception that had the Moon square to ascendant/descendant and the descendant was asc/desc was square my Moon. And the part of fortune at that time was conjunct my natal Vertex exact.
