איתמר, רב אמר: קושר, ושמואל אמר: חושב, ורב אסי אמר: יושב, אף על פי שלא קישר ואף על פי שלא חישב
“It was stated [that] Rav said: he ties [the branches together on Shabbat eve]. And Shmuel said: [If he merely] has in mind [on Shabbat eve that he wishes to sit on them on Shabbat, he need not tie them together]. And Rav Asi said [if he even briefly] sits [on them on Shabbat eve, sitting on the branches is permitted the next day], even though he did not tie [them together] and even though he did not have [that] in mind.”
Simply put, the Talmud cites three opinions concerning how a person must prepare an object for use on Shabbat so that it is not Muktzah: (1) he must do a formal act of preparation with the object; (2) he merely needs to think about using the object on Shabbat (that is, to mentally designate it for use on Shabbat); (3) he merely needs to use it once before Shabbat (with no formal act of preparation or mental designation). What is the Halachah?
(a) Tosfot cites Rabbeinu Shimshon who rules leniently (one merely needs to use it once before Shabbat), because that seems to be how Rav Asi concludes in the Talmud.
(b) The Rambam (Hilchot Shabbat 25:21) rules that one must think about it before Shabbat (the second opinion in the Gemara).
(c) Tosfot also cites Rabbeinu Tam who rules like the Tana Kama, that one must do a formal act of preparation. His ruling is based on a Talmud later in Shabbat.
So what is the Halakha? The Shulchan Aruch (OC 308:20) rules like the lenient opinion of Tosfot (a), that using it once before Shabbat is sufficient preparation for Shabbat.