I think you've got to bid for part time working, so it's done in seniority order, if it's available. There's probably some basic experience requirements too.
Practice area calculations in shapes, fractions, addition, multiplication, subtraction, division. If I remember correctly, there quite a lot of working out areas in shapes, quick divisions, etc but I did it 4 + years ago. There are some really time consuming questions, and I left those well alone.