Yevamos, Chapter Five, Mishnah 3. This mishnah continues to lay out the various scenarios of a get and a ma'amar, chalitzah and cohabitation, and the halachic result that follows from each combination.
"Nasan get ve'asah ma'amar" - he gave a get and then made a ma'amar:
As was mentioned in the first mishnah of the chapter, once the yavam gives a get, he disqualifies the yevamah from yibum - both for himself and for the other brothers - yet chalitzah is still required because of the zikah. Therefore, when he gave a get and only afterward made a ma'amar, "tzerichah get vachalitzah" - she requires a get and chalitzah:
A get - for the ma'amar. Even though a get was already given earlier, it was given before the ma'amar, and once he made a ma'amar an additional get is needed for it.
Chalitzah - in order to remove the zikah.
"Nasan get uva'al" - he gave a get and then cohabited with her:
Once the get was given, the yevamah became disqualified from the mitzvah of yibum with any of the brothers, and therefore the cohabitation that follows is an invalid cohabitation that does not properly fulfill the mitzvah of yibum, and the yevamah cannot remain married to him. Therefore "tzerichah get vachalitzah" - she requires a get and chalitzah: a get, because on a Torah level the cohabitation is effective, and so a get is needed from the yavam who cohabited with her, even though his cohabitation was invalid; and chalitzah, because an invalid cohabitation does not remove the zikah and does not complete the obligation of yibum, and therefore chalitzah is still necessary.
"Nasan get vechalatz" - he gave a get and then performed chalitzah:
In this case - "ein achar chalitzah klum" - nothing takes effect after chalitzah. This chalitzah is final, and nothing is required after it. The get does not call for any follow-up: all the get accomplished was that the mitzvah of yibum was ruled out, but the option of chalitzah remained in place. Once the chalitzah was performed, nothing further takes effect after it.
The Gemara explains that this expression, "ein achar chalitzah klum," teaches that even if he betroths her afterward, the kiddushin does not take hold. This understanding is based on the fact that our mishnah follows the view of Rabbi Akiva, who holds that a child born from a relationship forbidden by a mere negative commandment is a mamzer, and therefore kiddushin does not take hold with a woman whom it is forbidden to marry, even where the prohibition carries no kares but is only a negative commandment. And here there is a negative commandment forbidding one to marry the chalutzah, as it says, "asher lo yivneh" - since he did not build the house and did not fulfill the mitzvah of yibum, he can no longer do so. Since this is a case of a negative commandment, Rabbi Akiva holds that the kiddushin does not take hold at all. This is what the mishnah means when it says "ein achar chalitzah klum" - even kiddushin does not take effect after it.
Chalitzah or cohabitation performed first:
The mishnah lists two groups of scenarios:
He performed chalitzah first - and afterward made a ma'amar, or gave a get, or cohabited with her.
He cohabited with her first - and afterward made a ma'amar, or gave a get, or performed chalitzah.
In all of these cases the mishnah states "ein achar chalitzah klum" - nothing takes effect after the original, initial chalitzah. The same applies to the second group: nothing takes effect after that cohabitation. Although the mishnah does not spell this out, that is its intent. If the chalitzah was performed first, the chalitzah takes care of everything, and a ma'amar, get or cohabitation that follows is meaningless; and if the cohabitation came first and the mitzvah of yibum was fulfilled through it, then a ma'amar, get or chalitzah that follows is likewise meaningless.
In summary: The mishnah concludes by establishing that these halachos apply "achas yevamah achas leyavam echad" - whether with one yevamah and one yavam, "ve'achas shtei yevamos leyavam echad" - or with two yevamos, two widows, and one yavam. All the laws listed here apply equally in both scenarios.