A sense of humour is all that is required!!!!!!!!!!!!
No, seriously in simplified terms you need to have an understanding of CAP371 & the FTL scheme from the company's Operations Manual.
Apart from that a lot of LUCK getting in with the right company. Good Crewing Officers are hard to find as most of it is done by computers & the so called crewing officers who merely input info the system & hope that what they've done is right. If not then the computer tells them.