Maseches Bikkurim, Chapter 3, Mishnah 1. The Mishnah opens with a simple question: "Keitzad mafrishin es habikkurim" - in what manner does a person set aside the fruits of Bikkurim.
Going Down to the Field and Identifying the Fruit:
The Mishnah says: "Yored adam l'soch sadeihu v'ro'eh" - a person goes down into his field and looks over his fruit, and he finds the fruit that ripened first, that is, the first one to emerge on that tree, while it is still ripening and growing:
"Te'einah shebikrah" - a fig tree that has produced its first fruit.
"Eshkol shebiker" - the first cluster of grapes to emerge.
"Rimon shebiker" - the first pomegranate to emerge on that tree.
Tying and Declaring:
"Koshro b'gemi" - he ties a reed onto the branch as a marker, so that it will remind him that this is the fruit designated for Bikkurim once it ripens. "V'omer: harei eilu bikkurim" - he declares over it that it is Bikkurim. From that moment the status of Bikkurim takes effect upon it, except that one waits until it grows and ripens, and then brings it together with the rest of the Bikkurim to the Beis HaMikdash.
The Opinion of Rabbi Shimon:
"Rabbi Shimon omer: af al pi chein, chozer v'korei osam bikkurim me'achar sheyitol osam min hakarka" - even though he already declared them Bikkurim while they were still at the beginning of their growth, the first declaration is not enough. After they have been picked from the tree and taken from the ground, he declares over them again that they are Bikkurim, now that they have ripened fully and been harvested.
In summary: In this Mishnah we learned the procedure for setting aside Bikkurim: a person goes down to his field, identifies the fruit that ripened first - a fig, a cluster of grapes or a pomegranate - ties a reed onto it as a marker and declares "harei eilu bikkurim", and according to Rabbi Shimon he declares them Bikkurim once again even after he has taken them from the ground.