How are you supposed to do an eye test?

To start, I have had glasses since I was in year 2 (I can't remember how old that is, I think it's 6-7 though) and I am now 21. I know I have been having eye tests for years but I'm starting to think I may have been doing them wrong this whole time and no-one told me. I think this because even when I have my glasses on, things are blurry.

Here's what tends to happen after I have walked in and signed in and someone has come and got me for the eye test:

• When I go into the eye test room, you have your glasses on. This means I have a chance to see the board and know the letters.
• When they ask me to read the letters without my glasses on, I can't see even the biggest one.
• I know the shape of the letter and say the letter I think it is based on the shape of it rather than seeing the letter.
• When they ask you to read as small as you can, I strain my eyes and go to the really small one and read the letters based on the shape of the blobs that are the letters.

I've just always done it, I never considered that I wasn't supposed to do that till I saw somewhere (I can't remember where I saw exactly) that your supposed to relax your eyes.

I am due for an eye test so what should I do? Can someone give me an explanation on how I should actually do it? Should I tell the optician about this? I am so nervous about it now I don't want my prescription to be so much worse than it has been in the past because I have been doing it wrong my whole life.