I did it for a couple of years, usually twice a year for Mocks and the main exams. It was harder than I'd thought as you have to stand throughout to make sure you can keep an eye on all the students, plus walking up and down aisles for the same reason. There was always a Lead Invigilator who took charge of 'posting' us around the room, usually one at each side i.e. 4 minimum, plus the Lead. Also the Lead Invigilator was usually responsible, with one helper, for sorting the question papers (often there would be different tiers for one exam or a couple of different subjects being tested simultaneously, which can mean different timings. Again sorting that was largely their responsibility. That all sounds more onerous than it actually is. In practice it all works well (at least 90% of the time). You'll have to do some training, which in my case was a morning, mostly online with an opportunity to ask questions.
I actually found the time went quite quickly, as you need to be alert constantly. It helped that I was in a school where I was previously a TA, so was familiar with my surroundings.
Good luck - I loved it, particularly the camaraderie and support from the other invigilators. You're never on your own, even if you are in one of the 'other' rooms - that's the extra rooms sometimes used for students who have allocated extra time/use of laptops etc.