What you are growing is Cuphea or false/Mexican heather. It is not a true heather (Calluna species) but a small subtropical evergreen that provides a similar appearance in hotter, more humid climates than true heather will tolerate.

Pruning should be done in spring when new growth begins. This is a relatively low, spreading shrub and any pruning should be done with that habit in mind. Yes, you will ruin the form if you try to keep them into a rounded mound. You will have much better color on these plants if grown in some shade - too much sun tends to bleach them out and results in a chlorotic appearance.

Here is a link that might be useful: Mexican heather, Cuphea hyssopifolia

I'm totally new at this so....
what's the correct way to prune them?
I didn't want to make them rounded, I loved the fanned out shape they had. They just became too tall, so I cut some off the top. They also had some dead woody sides, so I cut them off too. That's how the shape changed.
Will they come back fanned out looking again?