From my own experience, although it does vary:
I am told arrive 10 minutes before appointment time. Every time I go I need drops, although not every one does. They sting like mad, but it soon eases. I am also asked a few general questions to confirm who my GP is and where I live and my date of birth. When the drops have had a chance to do their work (normally about 10-15 minutes) you are called backe and a camera is positioned in front of your face. Pictures are taken of each eye, you will probably be told to follow a coloured light.
If you are like the operator will be nice and explain what's going on and let you see the pictures, but lots of people are unlucky and get someone who just wants to get the job done or is a bit grumpy.
In a couple of weeks you will get a letter with the results. There are not normally any problems, but in the unlikely event there are, you will be advised what to do next.
I must emphasise take a pair of sun glasses with you or you wont be able to see where you are going
If the centre is out of the way get someone to drive you or take a taxi or the bus.