[Keybinding is important if you're interested in taking your game to the next level. It's important not only for hardcore players and PvP nuts, but also for casual players as well. If you're interesting trying it, but unsure how to begin, I'd like to quickly share the method I used to teach myself 4 years ago.]
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A: Start Slowly
Do not overwhelm yourself initially
Choose 2 abilities to keybind initially. Begin with either the two most often used abilities or reactionary spells. For instance, if you're an Arcane Mage you could choose Arcane Blast/Arcane Missiles or Blink/Counterspell. If you're a DK, you could choose Icy Touch/Plague Strike or your strike abilites, etc.
B: Where to Begin Binding
Keep in mind hand placement
The idea of a keybind is not only to avoid searching for and clicking a spell on screen, but also to be able to easily string abilities together with a sensible layout. This guide assumes you use ASWD for movement. The most easily accessible keybinds that do not require searching for keys are 1, 2, 3, R, F, C, X, Z and G. Choose two of these keys for your initial choices. Head to the target dummy and practice your rotation using these two keybinds until they begin to feel an afterthought. This shouldn't take longer than 15-20 minutes, but this will vary of course.
Alternatively, if you have a mouse with extra buttons that you can bind, it's an excellent place to bind reactionary spells. Choose one of these bindings for your first if you feel comfortable doing so.
C: Adding Subsequent Bindings
It gets easier and easier
Once you're feeling comfortable with your initial bindings, add at least 2 more at a time and head either back to the target dummies or just farm some easy mobs to form muscle memory on your new bindings. Don't overwhelm yourself by trying to remember 10 new bindings at once. If you dedicate just a couple of hours to this, you could find yourself with dozens of keybindings in your first session anyway. Once you begin using your keybinds in earnest, you'll begin loathing when you have to click one of your non-bound abilities and the urge to add more will become grow exponentially.
D: Test Your Bindings Ergonomically
Try different combinations
It may initially seem like a good idea to bind an ability to say the number 7. This is fine if it's a non-reactionary spell, but keep in mind that in the heat of battle, if Counterspell, Kick or Psychic Scream is bound to 7, it's not going to be something you can quickly and accurately find without looking at the keyboard 100% of the time. Stick long duration, non-combat spells in these places. Finally, remember that even if two keybinds are close to AWSD, they still may be hard to use in succession. For instance, you want to be able to easily use some spells back to back, so keep these abilities quite close and easy to reach if this is the case.
Thanks for reading. There are far more comprehensive guides around and I suggest you hit one of these if you want really in depth guidance. Please feel free to ask any questions here and I'm more than happy to answer them or find the answer for you.