What is the meaning of "Ve'asesah es Tzipornehah"?
Rashi and Ramban (both citing R. Akiva in Yevamos, 48a), Targum Onkelos and Targum Yonasan: It means that she must let her nails grow long 1 (Because, like letting her hair grow long, the Pasuk is referring to making herself ugly - Sifri).
Ramban (citing R. Eliezer in Yevamos 48a): It means to cut her nails. 2
Ramban: Because 'Asiyah' is an expression of 'fixing' - like "Ve'asas es ha'Tevu'ah" in B'har Vayikra, 25:21, and "Vaya'as Elokim es ha'Raki'a" - Rashi in Bereishis, 1:7. See also Ramban DH 've'Al Da'as Raboseinu'.
Ramban: As in Shmuel 2, 19:25 "ve'Lo Asah RagLa'av, ve'Lo Asah S'famo". See Ramban DH 've'Al Da'as Raboseinu'.
Why is the Y'fas To'ar commanded to do her nails and shave off her hair?
Rashi (citing the Sifri): In order to make her look ugly. 1
Riva: She shaves like a Metzora 5
To induce you to send her away and not to marry her.
Ramban: See Iyov 1:20, and Yirmiyah, 7:29.
Ramban: Which women tend to grow and paint.
Ramban: For having been separated from her father and mother - as the Torah indicates in Pasuk 13.
She also requiress Haza'ah [with Mei Chatas] like other captives, as the Gemara states in Makos (I did not find this in Makos, but in the Yerushalmi Pesachim, 8:8. It is an Asmachta (P'nei Moshe there), since live Nochrim can become Tamei only after conversion. (PF)
What are the implications of "Vahaveisah el Toch Beisecha"?
Kidushin 22a: It teaches us that he may not oppress her (by being intimate with her in the field). 1
Da'as Zekenim, Hadar Zekenim, Riva a Rosh: It means that one may only force her to change her garments after bringing her home after the war.
Riva: It implies that he should treat her like a wife and take her into his house (room), and not belittle her by being intimate with her in another room.
R. Tam (in Kidushin, Ibid.): With reference to the second time - but the first time is permitted.
