Gemara

(Mishnah): If one only scraped (made incisions in the skin) or only wrote (with dye, which gets absorbed in the skin), he is exempt. He is liable for Kesoves Ka'aka only if he wrote and scraped;

He is liable for writing with dye, mascara or anything that makes a lasting impression.

R. Shimon says, he is liable only if he writes ha'Shem - "u'Chsoves Ka'a'ka Lo Sitnu Bachem Ani Hash-m."

Question (Rav Acha brei d'Rava): Must he write 'Ani Hashem'?

Answer (Rav Ashi): R. Shimon does not refer to Hash-m's name;

(Bar Kapara - Beraisa): He is liable only if he writes the name of idolatry - "... Ani Hash-m", and no other (god is Hash-m).

(Rav Malkiya): One may not put oven ashes on a wound, because it looks like a tattoo.

Rav Bivi bar Abaye was careful not to put ashes even on a bloodletting wound.

(Rav Ashi): Wherever there is a wound, the place proves about the ashes (that they are for healing, so it is permitted).

Rishonim

Rambam (Hilchos Avodah Zarah 12:11): Tattooing of the Torah is to scrape the skin and fill the gash with mascara or ink or other rouge. This was the custom of idolaters, to mark themselves for idolatry, i.e. he is a slave sold to it and denoted for its service. Once a man or woman marks himself (or herself) in any place on the body, he is lashed. If one wrote but did not mark with dye, or marked with dye but did not scrape, he is exempt until he does both, for it says "Kesoves Ka'aka." This refers to one who wrote. If others wrote on him, he is liable only if he helped to do an action. If he did not do anything, he is not lashed.

Rambam (16): If Ploni tattooed Korach and Korach helped, if both of them were Mezid, they are both lashed. If one was Mezid and the other was Shogeg, the former is lashed and the latter is exempt.

Rosh (Makos 3:6): The Halachah follows Rav Ashi, for he is Basra.

Kitzur Piskei ha'Rosh (3:6): He is liable only if he writes the name of idolatry.

Ritva (21a DH v'Nir'in): The Rif rules like R. Shimon due to Bar Kapara's Beraisa (which is like R. Shimon) and Rav Acha. I disagree. This does not warrant ruling unlike the Rabim, especially since the Sugya about ashes is like Chachamim. R. Shimon would permit even mid'Rabanan.

Note: The Rif just brings the entire Mishnah and the Sugya.

Nimukei Yosef (4b DH Masnisin): The Halachah follows the first Tana.

R. Yerucham (Toldos Adam 17:5 p.159:2): Some rule like the opinion that he is not liable until he writes the name of idolatry, even if there is no wound. The primary opinion obligates when there is no wound even without writing the name of idolatry.

Tosfos (Gitin 20b DH bi'Chsoves): There is an Isur mid'Rabanan even without scaping and writing with ink. One may not even put oven ash on his wound because it looks like Kesoves Ka'aka!

Poskim

Shulchan Aruch (YD 180:1): Kesoves Ka'aka is to scrape the skin and fill the gash with sand (Gra - this should say 'mascara') or ink or other rouge.

Bach (1): Rashi explains that first he writes on his skin with an ingredient or ink, and afterwards he scrapes the skin with a needle or knife and the dye enters between the skin and the flesh, and it appears there permanently. R. Shimon agrees that it is forbidden to tattoo any writing, just he holds that there are lashes only for the name of idolatry. Rashi inferred from the Mishnah. R. Shimon said that he is liable only if he writes ha'Shem. He did not say that it is not called Kesoves Ka'aka (i.e. and is permitted) until he writes it. It is forbidden because also this is part of the pagan rite.

Bach (2): The Rambam discusses scaping and then filling the gash with mascara, but the same applies to writing with dye and then scraping, like Rashi says. The Mishnah exempts if he did only one thing, i.e. he scraped or wrote. It does not exempt one who wrote and then scraped. Rashi discussed writing first, like the Torah and Mishnah connote (Kesoves Ka'aka). The Rambam and Tur discussed scraping first, for this is how it is normally done.

Shulchan Aruch (2): If others wrote on Ploni, Ploni is liable only if he helped.

Shach (4): The Beis Yosef says that we learn from cutting the Pe'os. Therefore, Ploni is exempt if he did not help, but it is forbidden.

B'Tzel ha'Chachmah (5:82:2): The law of Kesoves Ka'aka on top of Kesoves Ka'aka is not clear from the Gemara or Meforshim. Machazik Berachah (OC 340:2) and the Malbim (on Vayikra 19:28, Sifra 76) compare Kesoves Ka'aka on Shabbos to writing on Shabbos. If so, Kesoves Ka'aka on Kesoves Ka'aka is exempt, like writing on top of writing, which is exempt on Shabbos, and all the more so when the latter writing is inferior and ruins the first. This is only if the latter dye is inferior; it should not be red. Iglei Tal (Ofeh 44:3) says that if the only change in a Melachah is how he did it (e.g. with his left hand), this exempts only for Shabbos, but if the Shinuy is in the Melachah itself, e.g. he planted in a pot without a hole or used hot spring water of Teveryah to cook, this exempts for all Isurim. If so, Kesoves Ka'aka on Kesoves Ka'aka is exempt. However, it is forbidden mid'Rabanan. The Minchas Chinuch (253:2) explains that we exempt one who tattoos his slave so he will not flee, for this is unlike the pagan rite, which is for the writing itself. If so, all the more so one who seeks to erase the first tattoo is exempt. However, l'Chatchilah one may not tattoo his slave. Perhaps the same applies to covering a tattoo. Semak (72) and the Bartenura obligate only if he writes (at least two) letters. Sefer ha'Chinuch, Pischei Teshuvah (1) and Machazik Berachah (340:3) obligate even for one letter. Some say that the Rambam does not require letters, but all agree that he requires a form. Mere scratches are not called Kesoves Ka'aka. Also for Shabbos one is liable only for a form. If Korach tattooed Ploni, Ploni is liable only if he helped. If Ploni wants to cover a tattoo, he can tell a Nochri to make mere scratches or dots without a form to cover it, and Ploni will not help at all. The Shach forbids even without helping, but Darchei Teshuvah (181:8) brings from Maharam mi'Rottenberg that he transgresses only going in the ways of the Nochrim, and it seems that the same applies here. Surely, erasing a tattoo is not the ways of the Nochrim! Normally, one may not tell a Nochri to do an Isur. The Magen Avraham (352:2) permits if there are three reasons why it is only mid'Rabanan. Here, he writes on top of writing, it ruins the first writing, and he makes only scratches. The Taz (OC 340:4) permits even if it is mid'Rabanan due to two reasons. If no Nochri is around, Ploni may ask even a Yisrael, especially if the tattoo prevents Ploni from putting on Tefilin properly. The Poskim rule like the first Tana, but Kitzur Piskei ha'Rosh and the Rif (according to the Ritva) rule like R. Shimon. The Aruch l'Ner cites a Tosefta (3:9, or 4:15 in some editions) that obligates only if he tattooed l'Shem idolatry. This is like Chachamim; they admit that one is liable only if done for idolatry. However, the Poskim connote unlike this.

Gra (1): The Tosefta is like R. Shimon. The Halachah follows Chachamim.

Minchas Chinuch (253 DH v'Hinei): The Rambam says that one who only marked or only scraped is exempt. This connotes that it is forbidden mid'Rabanan. However, the Rambam (Halachah 7) says similarly about shaving, and the Kesef Mishneh says that he exempts. Mishnas Chachamim says that the Rambam permits, for he does not mention lashes mid'Rabanan. This is not a valid inference. Tosfos in Gitin connotes that it is forbidden mid'Rabanan, and so rules the Beis Shmuel (124:16, regarding just scraping). I say that the Isur is only for a permanent scratch, but not for temporary writing.

Hagahah (3): Rav Sadya Gaon (Sefer ha'Mitzvos) says that the Torah forbids just marking or scraping, like Chetzi Shi'ur (doing part of an Isur).

Shevet ha'Levi (3:101:1): The Minchas Chinuch forbids any permanent marking on the skin, even with ink. There is no source for this.

Shulchan Aruch (3): One may put oven ashes on his wound.

Beis Yosef (DH u'Mutar): The Rambam did not mention this. It seems that he permits, like the Rosh. If he forbade, he would have mentioned it!

Bach (4): Rav Ashi needed to say that wherever there is a wound, the place proves (that the ashes are for healing), lest one ask that after it heals, a mark from the ashes will remain, and it will looks like Kesoves Ka'aka! He answered tis by saying that the place proves, i.e. even after it heals, there will be a scab and it would be clear that the mark was not Kesoves Ka'aka.