In Demon Souls you could just press select (I think) and add your selected message. I would like to know how to do it though because I like everyone's help and feel bad at not being able to provide back.
In order to provide hints you'll need the Orange Guidance Soapstone:
You can buy this from the Undead Merchant in the Undead Burg, near the first bonfire in the area, for 100 souls:
The soapstone behaves like a consumable item with an unlimited number of uses. So in order to use it you need to equip it to one of your item slots, than active it with "square" (on the PS3) to use it. This will give you the option to either write a message, delete one of your messages, or rate another users message.