Halakhah for Kiddushin 136:3

And let it not be difficult to you [that], if so that we do not say like the words of the translator (Onkelos) - from which verse do we learn the negative commandment of marrying a maidservant? And lest you say, from that which it is written (Deuteronomy 7:3), "Do not get married with them" - that is not [correct], as behold, it is explained in the Gemara that this is coming [specifically] for the seven [Canaanite] nations, and about them is it written. And [it is speaking about] specifically in their conversion; as in their gentileness, the expression, "marriage," is not applicable - and as we will write with God's help in its place, in the Order of Ve'etchanan (Sefer HaChinukh 427). But it is possible to say that we learn the prohibition of a maidservant as included in the prohibition of all the nations that are not Israelites, that it is prohibited to cling to them. And [it is] like the Sages expounded (Kiddushin 68b) from that which is written (Deuteronomy 7:4), "For they will remove your son from after Me" - to include all who remove. And likewise, a Canaanite maidservant is included in those who remove, since she is not a full Israelite; as behold, there are some commandments about which she is not obligated. And those are the ones in the section of the Torah in which it is explicitly written, "Speak to the Children of Israel" (e. g. Leviticus 18:2), to exclude anyone who is not from the Children of Israel, as it is explained in the Gemara in a few places. And with this, they are included in those that remove.

The laws of the commandment: That which they, may their memory be blessed, said (Kiddushin 21b) that [the law of] a woman of beautiful form [applies] whether she is a virgin or [is not] or [even if she is] the wife of a man. But it is not permitted to take two, as it is stated, "her" - not "her and her friend." And [also] that which they said (Kiddushin 22a) to not pressure her in the war, but rather he must isolate himself with her in a house - that he not think that the Torah permitted her in any fashion, and even publicly. And she must convert before he marries her as a wife. And after the conversion, he marries her with a marriage contract and betrothal, and the law that pertains to her is like the law of [all] the daughters of Israel. But if she does not want to convert, he puts up with her twelve months and sends her away. And after the month of crying, he waits for her two more months. And (Sanhedrin 21a) if she becomes pregnant from the first intercourse, the offspring is a gentile and not his son for any matter of all the matters - and like the Sages, may their memory be blessed, said (Kiddushin 68b), "Your son that comes from a gentile woman is not your son, but rather her son." And Tamar, the sister of Avshalom, was from the first intercourse of a woman of beautiful form, whereas Avsahlaom was born after the marriage. And it comes out that Tamar was the sister of Avshalom from [the side of] his mother, and [so] permissible to be married to Amnon. And so did it state (II Samuel 13:13), "please speak to the king, as he will not prevent me from you." [These] and the rest of the details of the commandment are in the first chapter of Kiddushin and in Sanhedrin.