I think these are the sort of instructions you're after but if you already know all of this stuff and I'm way off the mark, my apologies.

It works exactly the same way as bolding, colouring or changing the font of text. Basically you use the square brackets to enclose all formatting instuructions. To turn on a particular format you put a square-bracketed command at the beginning of your text, to turn it off you put a square-bracketed command with a / in front of it at the end of your text.

Semi-automated method:

1. Go to your journal and highlight the text you want to quote.
2. Hit alt-c to copy that text.
3. Go to the person's journal where you wish to answer the question.
4. When you've got the cursor ready in the reply box, click the "Wrap [ QUOTE ] tags" button
(you'll find it at the top of the message window, it looks like a little cartoon speech bubble).
5. Hit alt-v to paste the text you copied in step 2.
6. Click the "Wrap [ QUOTE ] tags" button again.
7. Type your reply underneath, then hit submit.

Manual method: (this is the old-fashioned way which I use because I'm not used to having little buttons to do it all for me):

1. Go to your journal and highlight the text you want to quote.
2. Hit alt-c to copy that text.
3. Go to the person's journal where you wish to answer the question.
4. When you've got the cursor ready in the reply box, hit alt-v to paste the text you copied in step 2.
5. Go to the start of the text you just pasted and type [ quote ] (without the extra spaces)
6. Go to the end of the text you just pasted and type [ /quote ] (without the extra spaces)
7. Type your reply underneath, then hit submit.

Hope that helps!