Quick tip: Be VERY careful at this step not to dislodge the tiny spring attached to the L2/R2 buttons. While removing the cover, it's easy to pull off either L2/R2 button. Inadvertently pulling off either button will also remove the tiny spring as well. While not too difficult to reassemble, it's not obvious where the spring goes as the spring is almost completely hidden along the side of the button (on the inside).
I just successfully completed analog stick replacements for two controllers. I completed the first repair without any trouble but managed to dislodge (and almost lose) the spring on the second time around. Needless to say, I spent some time trying to figure out where the spring is supposed to go (no photos of this anywhere). By sheer luck, I figured it out on my own. :)
Good luck and take your time--especially the first time!