I'm also not certain but last month I started checking and I fell pregnant (which sadly wasn't to be) so I got a rough idea.
Checking your cervix is when you insert your finger and feel the very back if the vaginal passage.
At the back is a blob of skin which is your cervix. If you google it you can get a rough idea of what it looks like.
A high cervix is when you can just reach it with the tip of your finger. When its high it should be soft so almost fleshy and like prodding your cheek. This usually happens around ovulation.
When af is nearing, your cervix will be quite low and hard, like your chin. You will feel it as soon as you insert your finger.
Now for the bit I still have trouble figuring out... The open/closed state. During your cycle and pregnancy your cervix should be closed, this feels like a slit. During af it will be open. You will most definitely feel a hole (bit like a man's jap eye) but do not put your finger in it! When it is open, this allows for blood to go through.
Make sure your finger is clean and keep nails trimmed as it is a very delicate part of your organ. Also this is a more accurate way of seeing what your cm is like.
With regards to time of day, I am not so sure but I did it a few times a day in various positions. Sat on the toilet is worst as it pushes your cervix down anyway. I found the one leg up on toilet seat best. I think its best to keep your body straight.
I've resisted from checking this month because it just worries me as I'm still unsure if findings. X
