With a click macro, you have a master macro which you spam.

It calls several other castsequences.
Each castsequence is called at the same time.
But because of the GCD, they execute top down.
So anything, in any of the macros off of the GCD will fire off immediately.
But the first thing on the GCD then prevents anything else on the GCD from firing.

Because of this, we can prioritize our spells/abilities.
By putting the Immolate before the Conflagrate, or Flame Shock before the Lava Burst, etc.

We need to know our personal spam speed.
And the duration of our abilities (or their cooldown, until they are reusable).
Translate this delay into seconds, and multiply by our clicks per second.
So we insert commas, which serve as "no action on this press", equivalent to the desired delay.

When we spam our master macro.
It executes the first ability.
And that ability will execute again, when the desired time (determined by spam speed/commas) passes.
Each subsequent ability also fires off... initially as the GCD allows, and then according to our timing.

And because everything executes top down...
If two things expire at once, they fire off in the priority sequence as well.

Each sequence will have a final ability, with no cooldown or dot component.
This will fire off when everything else is on the comma stage.
So our click sequences auto adjust for haste changes.

Once made, we tweak them (add or remove commas) by visiting a target dummy.

And we end up with one-button spam for almost any class.
Which makes boxing any dps close to as optimal as one-boxing.
We miss out on specials when a proc occurs.
But can work these into the dps too, but as something in addition to /click.