The Torah states that the Hadas must be braided. This means that there are three leaves that extend from each area of the branch and that these three leaves are symmetric to each other that they extend from the same horizontal line. If two leaves are symmetric but third leaf grows higher or lower than the other two, it is not considered braided. This form of Hadas is called a Hadas Shoteh and is invalid for use throughout all seven days of Sukkos. It may not be used even in a time of need that no other Hadas is available, even if one desires to take it without blessing.
How many leaves on the Hadas must be Meshulash? Initially it is a Mitzvah to search for [i.e. purchase] a fully Meshulash Hadas. This means that the Hadas should be Meshulash with three symmetric leaves by every set of leaves for its entire Shiur of three Tefachim. Nevertheless, even if the Hadas is not entirely Meshulash, if it is Meshulash for majority of its length then it may even initially be used with a blessing. This means that if majority of the sets of three leaves coming out of the branch have their stems coming out from the same horizontal line throughout majority of the Shiur of three Tefachim, then the Hadas is valid and one may even initially recite a blessing over it. [This validation applies even if the head/top leaves of the Hadas is not Meshulash. If, however, majority of the Hadas is not Meshulash throughout its Shiur of three Tefach, the Hadas is invalid.]
Must majority of the Shiur [i.e. 24 cm] be Meshulash or majority of the entire branch?
If the Hadas is is seven Tefachim long and is majority/fully Meshulash within its Shiur of three Tefachim, but is not Meshulash for the remaining four Tefachim, some Poskim rule we follow majority of the Shiur of three Tefachim and the Hadas is thus valid. Other Poskim however leave this matter in question. It is clearly implied from Admur like the former opinion, that the Hadas is Kosher.
Must the majority Meshulash leaves be in order?
It is not necessary for the majority Meshulash leaves to be in order.
 646/3; Michaber 646/5; Tur 646 in name of Rosh 3/10; Raavad in Tamim Deim 228 and on Rambam 7/2; Levush 646; Chayeh Adam 150/2; M”B 646/18
Other opinions: Some Poskim rule that if the entire Hadas is not Meshulash it is invalid. [Geonim, brought in Tur ibid; Rav Hamagid on Rambam 7/2] Other Poskim rule that so long as there is even one set of Meshulash leaves the entire Hadas is valid. [Baal Haitur and Rambam, brought in Tur ibid] See Kaf Hachaim 646/36
 Rama 646/5; Tur in name of Rosh ibid in name of Ravad; Vetzaruch Iyun why Admur omitted this matter
 Michaber ibid; Taz 646/9
The reason: The area of the Hadas that is not Avus [Meshulash] is considered to be non existant and hence the Hadas loses its Shiur. If however its majority is Meshulash then it does not lose its Shiur. [Taz 646/9]
 Kashrus Daled Minim p. 114; Piskeiy Teshuvos 646/6
 Bach 646; Bikureiy Yaakov 646, brought in Biur Halacha 646 “Leikuva”;; See Taz 646/9 “When it is not Meshulash it loses its Shiur” thus implying that we only care about the top three Tefachim
 Peri Megadim 646 M”Z 9; See Biur Halacha 646 Leikuva who brings the opinions in this matter and does not give a final arbitration; Shevet Halevi 7/2 and Piskeiy Teshuvos 646/6 concludes to be stringent
 646/3 “It is entirely Meshulash throughout its Shiur of three Tefachim….Majority of its stest are Meshulash for its Shiur of three Tefachim..”