At low levels, I just go with a single spell.

Once I have two or more spells I want to bother casting... which might be quite a few levels, as 5x Smite is enough to kill most same level stuff for a very long time, I move towards the click castsequences.

Something like Smite as the fall through.
Initially I might even have a click castsequence, but nothing in the other slots, just the fall through.
But eventually add say Shadow Word: Pain, Mindblast and Holy Fire.

Most of my click castsequences are 3-5 clicks plus a fall-through.
Let's say you go with Click 1, 2, 3, 4, 5 and 6.
Put the fall-through on button 6.
And you have the option to put other macros (castsequences) on the other buttons.
And then drag them into and out of your overall macro, by moving them onto or off of the buttons clicked by the master macro.