I originally found the leveling instructions and recommended herb amounts in the slightly out-of-date Inscription Guide from Siha at Banana Shoulders. The guide was first written during the PTR before patch 3.0.2 (when Inscription was first introduced to the game). I tried to get as close to the amounts Siha suggested as I could before I went mad from flower-picking.
When I finally started actually leveling Inscription, it turns out that the numbers of herbs recommended in Siha's guide are really high compared to what you actually need. I'm guessing they either increased the drop-rate of pigments from milling, or they reduced the number of inks needed to level up to each new training level.
I built this quick-reference shopping list chart similar to my Herbalism Chart, so that I could see which herbs are in which pigment group, and the herbs that mill into certain pigments and inks.
Click image to make bigger!
The numbers of herbs in the chart are approximately the numbers I used in my leveling process.
Now it turns out that once patch 3.3.3 goes live, many of the glyphs that require two inks to make will only need one. So you will need even less herbs than I have in my chart! I'd estimate about two-thirds of the values I used will be needed after the patch. Of course I am over the moon about this, especially since they decided to announce it after I was already over 400 inscription (/grump).
During the leveling process, try to make orange and yellow glyphs and scrolls to level up. Make sure you make the rare inks such as Hunter's Ink and Fiery Ink as they become available and before they turn gray. Also, make sure to train every five levels to get the next set of orange glyphs.
Once you reach 75 Inscription, Minor Inscription Research is available. This 20-hour cooldown spell teaches you one minor glyph, until you have learned them all. In Patch 3.3, there are currently 35 minor glyphs in the game.
At 385, Northrend Inscription Research is a second 20-hour cooldown through which you learn the remaining major glyphs that can't be learned at a trainer.
As you level up, blue and epic off-hand items become available to craft. The mats for these items are usually fairly expensive but once you get to the higher level items they usually sell well at the Auction House so check out the values of mats compared with the finished product on your server to see if making them is worthwhile. The epic items [Faces of Doom] and [Iron-bound Tome] can sell for hundreds of gold on my server.
At level 400 Inscription, the Master's Inscriptions become available. These are the replacement for the Sons of Hodir reputation shoulder enchants, and was often the sole reason people level Inscription in the first place. Now it's not quite as arduous to get to exalted with the Sons of Hodir, but I'm happy to not have to bother with that rep grind.
And that's pretty much the deal with Inscription.
