There are three ways of doing it:
1. Copy the text you want to quote, and then put it between "quote tags".
You want something like
[ quote="Keef" ] text being quoted [ /quote ] (but without the spaces I inserted around the [ ] )
which results in
Originally Posted by Keef
text being quoted
2. Click on the "Reply" button. In the URL box at the top of your screen you will see something like
http://www.pprune.org/newreply.php?do=newreply&p=8644874&noquote=1
Remove the 1 at the end of that and when you post, the whole quoted post will appear. You will want to edit out the irrelevant bits.
3. ian16th's way, which I never got the hang of.