I've got a question quite specific, and directed to expert pilots: How to improve a manual skill during solo flights? I understand that take an instructor with me would probably be the best way, but I need to save money for the incoming CPL course. So any suggestions?
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.
For this, there is just no substitute for practice. The phrase "to improve manual flight skill is a pretty broad request for a large variety of basic airmanship skills so the only way I could help answer this with a few paragraphs is to concentrate on one specific skill.
Remember that a pilots license or rating is just a journeyman's license to practice basic airmanship; flying an aircraft is an art which takes a lifetime to master. There are pilots with over 20,000 hours total time which learn something new on every flight.
I might recommend the following general tips for you:
Practice! Practice! Practice! Work on specific maneuvers and skills which you are struggling with and perfect them.
Consult the FAA's books such as the Airplane Flying Handbook or the Pilot's Handbook of Aeronautical Knowledge for information on good, basic airmanship. They're very well written and contain a wealth of knowledge on these subjects. Best of all, they're free online!
No man is an island; talk with other pilots and flight instructors about these subjects and learn from them. Build up this ecosystem of knowledge and always keep expanding this base.
Keep current! Fly on a regular basis and practice different skills e.g. Maneuvers, navigation, pattern work, etc. and don't forget about flight reviews. Do a BFR every or an IPC year with an instructor, whether your current or not. It will help weed out bad habits that slowly grow into your flying routine.
Set personal minimums and periodically evaluate these for safe operations.
Use simulators to perfect techniques once an instructor has demonstrated them to you in a real aircraft. FSX, X-Plane, Prepar3d, etc may be games but they use extremely accurate flight models and can be used to perfect a lot of basic airmanship. Note: fly the techniques your instructor taught you in the sim; do not self teach. You can get a lot of bad habits ingrained if you do not.