How to improve manual flight skill?
Don't use the autopilot, use the trim, and look outside!  Fixating on the instruments will not make you better.  Know the sight picture out the windshield and what wing looks like in relation to the horizon.  Set 25' altitude brackets and 2° heading brackets and hold yourself to them.  It'll seem impossible but you'll get better.  Set those brackets with the intent to hold them by looking outside, not chasing needles in the cockpit.

Fly using the actual horizon as your reference, which is safer anyway, and make minor adjustments based on a periodic instrument scan that should become less and less frequent as you get better (think 10-15sec between scans to start).  Hold heading based on distant objects, not the compass or DG.

Finally, use the trim.  If it takes more than two fingers to adjust pitch, you are not trimmed.  If you fly a plane with a trim wheel between the seats get used to resting your hand on it.  Every time you change the power setting, retrim.  In an updraft?  Retrim.  Down draft?  Retrim. It'll become second nature in no time.