Electrons fill orbitals singly before they start sharing.
The 4s subshell usually fills up first because it has lower energy than the 3d subshell.Once the 4s subshell is full, the 3d subshell starts to fill up.
The 3d orbitals are occupied singly at first. They only double up when they have to.
The exceptions:
Chromium prefers to have one electron in each orbital of the 3d subshell and just one in the 4s subshell — this gives it more stability.
Copper prefers to have a full 3d subshell and just one electron in the 4s subshell — it's more stable that way.
Writing the electron configuration of a transition metal ion:
Write down the electron configuration of the atom.
Work out how many electrons have been removed to make the ion by looking at the charge on the ion.
Remove that number of electrons from the electron configuration, taking them out of the s-orbital first and then the d-orbitals.
