Code:
/use [mod:alt] @Player] Flash Heal; [harm] Mind Flay; [help][] Flash Heal
Code:
/use [mod:alt] @Player] Prayer of Mending; [harm] Shadow Word: Death; [help][] Prayer of Mending
Macros are just fancy if/then statements.
[mod] is the modifier key condition, if true then @
Player] is the target of the spell.
If alt isn't down, [mod] is false, so the macro moves to the next part. Think of ";" as "else if".
[harm] is an enemy condition. If your target exists and is something you can harm, then do this spell.
[help] is the opposite. Here, though, we have []. This is a catchall conditional that always evaluates to true. Use it for the last spell of a macro if you want that macro to behave like that spell in all cases the macro doesn't match. In this case, you asked for the macro to use Flash Heal without a target and no alt key, so [] is the easiest way to do that.
The second macro with Shadow Word and Prayer works the same way.