[ah7391]

What if I had, let's say, Se3+ ?

Ground config of Se:
Se: [Ar]4s2 3d10 4p4

Where would I start "taking off" electrons (on the s or p shell)?

[ramboacid]

You'd remove the outermost electrons first, so the p-orbital electrons would be the first to go. If you kept removing electrons then you'd later remove the s-orbital electrons.

[Sophia7X]

and in case you come across a transition metal cation, there's a little irregularity with the electron configuration. You take the s electron away before the d electron.

[juanrga]

You would start with the p