You buy herbs from the Auction House (or gather them, obviously) and mill them to get pigments. Then you use these pigments to craft inks, then you need one (or two in a few cases) ink and a parchment bought from the NPC to craft a glyph. Most people just buy the herbs from the AH, craft all the inks themselves, and level like that ... Bad idea.
A few patches ago, Blizzard introduced an "Ink Trader" named Jessica Sellers to the game, this NPC will let you trade the Ink of the Sea you get by milling any Northrend Herbs for ANY lower level ink. If you want to compare it to other professions, just imagine it as a NPC that will give you Iron and Mithril in echange for Saronite.
The easiest way to level Inscription is to buy the first tier of low level herbs (because stuff like Peacebloom or Silverleaf is usually really cheap) and then buy Adder's Tongue, Lichbloom, Icethorn.
These 3 herbs yields the highest amount of pigments to create the Ink of the Sea you will need to buy the lower level inks. You can always check the AH every now and then for the lower level herbs if you think it's a cheaper alternative but on most servers, it won't.