Half equations? (Chemistry)

[masterex567]

Can someone please briefly explain how to write half equations in electrolysis for Chemistry?

Here are some examples to try at:
1. Molten Magnesium Fluoride
2. Molten Aluminium Oxide
3. Aqueous Copper II Chloride
4. Molten sodium chloride

I don't understand where to add, or subtract an electron or where a single or two electron are supposed to be subtracted.
Please helpp