Hund's rule states that the electrons pair up only after each orbital in a sublevel is occupied by one electron.
This means that first one electron occupies an orbital in the sublevel and only after that, pairing starts to occur in that orbital. For your understanding, I have given an example to show how pairing of electrons occurs in, say the orbitals of “2p” sublevel.
↑ ↓= electrons
(Note that upper and down arrow represent electrons with opposite spins (+/-) as no two electrons in an orbital can have the same spin).
2p can accommodate 6 electrons in 3 orbitals. Hence each orbital gets 3 pairs of electrons.
_ _ _ (The _ represent the 3 orbitals of 2p)
↑ _ _ (First electron in first orbital)
↑ ↑ _ (Second electron)
↑ ↑ ↑ (Third electron)
↑↓ ↑ ↑(Pairing of electrons starts in first orbital)
Hence pairing occurs after each of the three orbitals get occupied with one electron.