The easiest way currently would just to use the stats for animated objects and golems.
Other than that, you can modify a non-construct creature by giving it traits common to constructs - the GMG section on creating creatures says that constructs generally have;
The GMG outlines how to make new creatures, and lists a bunch of traits for each creature type (that most creatures of that type have). For constructs this is that many of them have the mindless trait, and a looong list of immunities.
I would make a template look kind of like this;
Traits: The creature gains the clockwork, construct and mindless traits, and loses any other creature type traits (such as beast or dragon or plant) that it previously had.
Immunities: bleed, death effects, diseased, doom, drained, fatigued, healing, necromancy, nonlethal attacks, paralyzed, poison, sickened, unconscious, mental
Weaknesses: The creature loses any immunity or resistance to bludgeoning and electrical and gains weakness to bludgeoning 5 and electrical 5
Breath Weapon: If the creature had a breath weapon, it becomes a steam breath weapon that is a cone and does fire damage.
Wind-up: The clockwork creature is powered by a wind-up device. It normally has the unconscious condition, which can only be removed by winding up a key attached to its back. Winding up the key is a 3-action activity with the manipulate trait and removes the unconscious condition for a duration. Smaller creatures generally have a shorter duration. Winding up the key again resets the duration but doesn't increase the duration further. When the duration finishes, the clockwork returns to being unconscious.
Overclock (1 Action): The creature gains the quickened condition for 3 rounds, but the remaining duration of being wound up is reduced by half of the maximum wind up duration.
As the bestiary usually says when it lists a template, it is always recommended that you are creative in applying it and add/remove abilities that fit/don't fit.
You might want to browse Pathfinder First Edition's clockwork stuff for inspiration. That link is to a random clockwork monster; I find the stuff below the actual monster entry more interesting---that's where the general blurb/rules for clockwork critters are.