Assuming it's true & which only a lifetime study would prove - if you're going to be sure of success, ensure it's a virgin bat and that it's hymen is 'traditionally' ruptured, though by a male bat of the same genealogical bloodline and that's at least three [ideally >4] times the age of the female.
The blood must be imbued with sage and ammonia in a plain copper basin - like a wok but without handles, more like a cymbal - it needs to be set in a heliotropic mineral bed of finely ground stone and it is essential that a church bell can be heard at the site where, and when, it is performed, or a tone from a bell must be constant throughout. Only use a glass ladle with an acid etched [frosted] handle and do not allow it to come into contact with the basin, nor should you pour from the basin into the ladle.
The absolute imperative is that the girl's umbilical cut is healed - some practitioners wear the umbilical as a garland, but this is deemed purely symbolic - and that there are no open cuts or scratches on her body of any kind - and that if bathed while awake, she must not utter a single sound nor be prevented from making one.
Of the elements, there must be no sulphur present, anyone with a skin condition must not attend, even a nervous tic that starts before the procedure is finished will cause failure. Any contagion present - even a wet/damp surface must be excluded from the prepared chamber.
...and finally - all from failed attempts;
- if there's any hair of either velous, terminal, or pubic type, even that of another person, anywhere on her body - it will fail.
- if the girl is asleep and has ever seen a bat, a picture of a bat, heard the noise a bat makes in flight & with it's sonic location - it will fail.
- if any reference to procuring bat blood is heard by the girl - specifically the words 'bat' & 'blood', it may fail and the degree of success is comparable to any future desire she has to grow hair upon her hairless body - if she associates bats & blood with hairlessness, then it will have failed.
Last of all - the blood has to be HER body temperature, not the bat's nor the room's and to bathe successfully you need at least the same amount as there is embryonic fluid in the girl's mother's womb at full term.
To warm the blood properly you must heat the powdered stone without imparting any soot into it.
If you use an electric hob/source to heat the stone - the supply must be cut & isolated, also excluded.
If the stone cools and is reheated, any condensation created will contaminate the 'theatre' and to continue will result in failure.
Always test on a small area first, if the product gets in the eyes rinse with plenty of water and drink fresh milk then consult a social worker who will ensure the child is given at least the same level of neglect in a state run care-home.
If albinoism occurs, phone the careline on 0255 255255255
If you failed to see where this was going from the off, please don't blame people for knowing the kind of response the question would elicit, nor the people answering it for expressing varying degrees of humour - just as those saying it's disgusting/they're disgusted still clicked the link instead of just ignoring it with the same level of disdain they have iterated by way of a response.
If you need further snappy disclaimer type literature, try reading a real legal & binding one in the style of ~, say Futurama, like all this is fascinating but so yesterday yet not without a lengthy wrangle in the law courts, some subplot skullduggery and a few hundred years stasis to wake up from.
If in any doubt at all, always consult a qualified shady ritual practioner with the Pewter Pentangle Certificate of Alchemical Nonsense.
May you enjoy many happy fulfilling years of parenting no matter what people think to your obscure lifestyle and how your strangely bald daughter[s] are turning out.