Loosen the mounting bolts for the caliper, slip a baseball/business card between the outer (moving) pad, and the rotor, and then snug the adjustable piston up. Not tight, just snug. Tighten the mounting bolts, back off the adjustable piston, remove the card.

At this point, you fine-tune the brake. The adjustable pad should be just not touching when the bike is loaded, while the opposite side should be adjusted until you get good bite at the lever (usually with the straight portion of the lever blade parallel with the grip, as it is pulled). Adjust to your tastes.

Assuming the rotor is straight, you're done. But don't assume the rotor is straight. It is not awful to true a rotor, but it may be significantly more frustrating than adjusting the caliper, if you haven't done it before. Using the caliper as a gap gauge, watch as the rotor passes through. Slight bends may be gently coerced out with the use of an adjustable wrench, fingers, purpose built tool, etc....

Bicycle owner's manuals have very rarely contained any useful information for setting up/maintaining the equipment on the bicycle. Some bikes come with parts manuals, which may be passed on to the purchaser, but many shops, in my experience, simply throw them out with the rest of the recyclable packaging.

I have found very few truly bad mechanical disc brakes, though they are out there (like most poor performing parts, found on *mart bikes...). 80% of them can be set up acceptably as above--of the remaining 20%, 19% might be brakes that have two adjustable pistons (easier to setup), while only 1% are truly beyond any saving. Remember that, for any braking system, everything works as a team. You must have low compression housing, smooth running cables, attached to a decent lever (the requirements here are not that high...), and of course, a properly setup caliper. Any one of those parts being out of sync can cause poor performance.