Just replaced the front brake of my bike, its terrible. It can barely even stop the wheel against me pedaling. The back brake is still using the original pad and doing a significantly better job.
All I did was remove the old one and put the new one in. Adjust the brake position slightly to avoid rubbing when the wheel spins.
Does it take time for a new brake pad to wear in at all? Is there something else worth trying? Its terrible currently.
You can also recenter the caliper by loosening the caliper bolts, then spin the wheel, grab the brake, then while still holding the brake down (or having a friend/second set of hands do it) tighten the caliper bolts again.
I did try that, got mostly aligned but had to manually nudge it after to stop it rubbing slightly. Think it might be worth cutting out a shim from a drink can and sliding that down each side of the rotor between the pads and doing the same thing, that should ensure an air gap on each side.