I got this other day from a post and saved it to my computer for future refference. Kinda already mentioned in the post but I will share it anyway. And it is from several different people, so I am sorry if I am qouting you and not giving them credit. It is from the post "Need help whit Healing"

Replace "char1" with a character name and bind this macro to F1 for instance. Make sure SHIFT+F1 and CTRL+F1 is unbound. Pressing F1 will shield the character, pressing SHIFT+F1 will cast Flash Heal and CTRL+F1 will cast Power Word: Shield.

You need to create the macro first of all and then you can drag the macro to one of your action bars. After that you need to do a key binding. If you hit ESCAPE and select Key Bindings you will get a list of all the available actions and the keys bound to them.

So if you drag the macro into the 1st slot of your bottom right action bar then go into key bindings and look for "Bottom Right Action Bar" you should find it in there somewhere, all 12 of them. Click on the 1st key box next to it and press F1 which should bind the F1 key to that action bar slot.