Turn so the apparent wind
is about 140.
Release the preventer.
Make sure the vang is on.
Depending on conditions, maybe turn down to 160 apparent.
Sheet in the main halfway. But no more than you are comfortable with.
Turn to dead downwind.
Sheet main in all the way.
Pause for a moment to see if the main will flop over on it's own.
Turn the boat to 140 on the other gybe.
Presumably the main will have flopped over.
Ease and trim the main sheet.
Attach the new preventer.
This all varies with conditions.
The vang is important. Having it on helps the main flop over.
If it's blowing hard you'll want to not delay easing the mainsheet.
But if it's blowing hard...shouldn't you have a reef in already?
If it's light you might do the turn and sheeting in one simultaneous operation.
Consider controlled chicken-gybes.