To add to the good advice already given.....

I made a fence last year using the same materials.
Cutting the concrete posts to size is not advised and I've never heard
of it. To keep the heights the same I just made sure the gravel board
was totally level with a spirit level (prop with bricks if need be)
and then measured the height of the post to be fixed from the gravel
board Digging the holes is the worst bit espcially if there are loads
of old stones, bricks and tree roots in the way - it can labour even
the strongest man. A quicker way (although I have not tried it) is to
hire a petrol hole digger - I intend to try one the next time.

I used quick set post fixing mix as well - absolutely essential. Used
to have some smashed up bricks handy as well to throw in with the mix
to add some strength. Never needed to support the posts, the mix and
bricks held the posts in place firmly after a few minutes of holding.