Hi, I'm an EO.
I do mocks x 2 for Yr 11, plus until covid, i arranged core exams for all other year groups too. I also sort out BTEC admin, entries for GCSEs and for the last three weeks have had daily arrivals of exam papers which go into secure storage (a room with filing cabinet with so many keys and locks it could secure the crown jewels).
You need to know a lot of guidelines - so as PP said, Google 'JCQ Instructions for candidates' and digest as much as possible. Plus also safeguarding.
You will need to train your invigilators every year. I have done three training sessions this year to make sure everyone is up to scratch since the pandemic break.
I do all physical admin - putting up signs, moving exam boxes of stationary around, moving piles of mock papers around, pinning up timetables and seating plans.
You need a good relationship with everyone in the school, especially repographics, office reception, deputy head, heads of departments AND most importantly the site manager who will be putting out your exam desks and move all the cardboard from your seventy billion exam paper boxes!
It is totally crackers from now until the end of June but it can be a lot of fun, very flexible during the year and quite rewarding when you see your extra time pupils working so hard.
I also have an emergency biscuit tin for pupils who are in tears outside an exam room. I promise them a chocolate biscuit for going back to an exam and it works wonders and they need the sugar after all the stress. (There's no rule about food in exams though obviously we don't advertise this to pupils)
The ability to talk to a crying 16yr old girl whose boyfriend dumped her on the eve of her Maths GCSE is a key Skill too!
Good luck OP I love my job.