Timing, rhythm, length, and speed are to distance control what impact angle, alignment, and stroke path are to direction control. Now we're talking precision versus force.
Elements of Direction Control
Putting is a series of connected events, which must all be executed precisely and consistently for success.
Often golf can be compared to pool, where the form, eyeline, and strike must be perfectly calculated. Hitting the cue ball dead center results in a straight shot, but off-center will create drastic deviations. The relationship between the golf club face and the ball is similar to that of a cue ball and stick.
The angle and location of impact determines the spin, direction, and lift of the ball.
The face angle and positioning of the club on the ball at address and impact is responsible for most of the precision and errors in a putt. It accounts for 83% of significant influence on the direction the putt takes - especially shorter putts.
In a putt under 6 ft, a divergence of even a degree or two will result in a missed hole.
The bottom line is that the center of the putter face, ideally indicated with a marked line on its face, should line up with the total center (left, right, up, down) of the ball. If there is a minor depression where the ball sits at tee from the levelled ground, a point of impact slightly lower than center (but still centered on left-right axis) could help with a tiny lift to compensate for any interference/resistance.
Body Alignment & Set Up
Body alignment is vital to control of the stroke path and consistent angle of impact. While a neutral posture set-up is standard and most likely to create a straightforward putt, the important thing really is that your preferred form is repeatable and complimentary to the other direction and distance control elements.
A neutral position looks like the nose, toes, and shoulders facing perpendicular to the target path line (from tee to hole). This also means the center of the putter face is aimed directly in front of the hole in a straight line, and sits behind the center of the ball.
As the stroke path gets followed, there should be a stable rotation in the upper spine, with a locked symmetry of the hands-forearms-shoulders. A neutral fitted putter will help with the maintenance of form.
Alignments of shoulders, eyes, and knees can vary based on comfort and compensation for natural arc/twist tendencies, as long as it's a posture that can be consistently recreated with precision results and is comfy.
Stroke Path & Arc
Stroke path refers to the invisible line that is drawn through the air by the club face and continued by the ball, in relation to the target, created by back & forward swing, arc, and transference of momentum.
Noticing any consistent patterns in your path may help to identify natural tendencies towards a left or right default. From there you can either identify the issue through trial and error/deduction (alignment/rotation/arc, stroke length, impact angle, visual illusion, ball placement, club/grip fitting, arc etc) or you can just adjust an aspect to compensate for the tendency.
The swing path plays a 17% influential role towards the overall success of the putt into the target hole. Even 5-10° divergence in path, with a perfect center impact, in a short putt can mean the difference between a sunken hole and a miss.
A natural arc of 15° is completely normal and even helpful. This is because the putting handle itself sits at an angle (70° typically) and in order to have zero arc, it would have to be positioned at a 90° directly over the ball which is an absurd way to expect anyone to putt.
The key to a predictable and relatively straight stroke path lies in proper levelled alignment and calculations accounting for your natural variations from the normal neutral default (crooked tendencies). When everything is lined up compatibly with you and is stable and controlled, the path is likely to go in the direction you want.
Setting up pegged gates along the path you'd like to follow is an amazing way to train your body and movements. Eventually the movements become natural and the tools can be removed.
Common Errors and Their Solutions
Learning to recognize the issues behind misdirection and applying an appropriate solution is going to mean you can troubleshoot any issue. Some things have multiple possible answers and certain elements which deviate from the neutral can be compensated for/cancelled out by overcorrecting other aspects.
Impact Angle Is Off
An impact angle that keeps veering left, for example, can be corrected with a minor positioning adjustment the other way with the body. Switching putters to one with a center notch on the club may also act as visual cue.
Alignment/Arc Path Is Off
An open body alignment/curved path left, for example, can be corrected by striking the ball slightly headed right (open putter face) to even things out. Don't forget to take into account significance (angle plays a more powerful role than path/posture, so posture changes must be more dramatic than impact angle adjustments).
Problems with set up can also be trained correctly using gates to guide the stroke and arc path, so the right swing becomes automatic.
Chances are if the arc is off, it's due to bad set-up posture. Poor form can be the general result of bad balance, lack of flexibility and strength, and too much movement in certain symmetries between address and impact. Having a routine workout to improve agility and strength, using a training tool like the Pro Path Putting Mirror to keep body parts level, as well as doing practice drills, can make a difference.
If you're seeing inconsistency in putts, and most elements have been troubleshooted, it's possible that the form just isn't comfortable or natural to you and needs to be more sustainable for repetition. Try experimenting and compensating for different approaches. The aim is a consistent technique that works - whatever that looks like.
You may also need to look into a different club head, grip, or shaft that fits you at better angles.
You can have all the technique in the world but if your judgement of 'straight' is off, then your aim will be. Whether the issue is eye problems, visual illusions of a straight line, inability to perceive depth and climate variations, there's some help.
Using string to connect point A (tee) to point B (hole) will help you calibrate whether you perceive straightness a little crookedly. Using gates to guide that path along the string will help with correction and cell memory. Correcting eye problems with eyewear can also help immensely and visual aids like the ProVision Rangefinder help with landscapes.
When in doubt, move the ball placement a touch, experiment with different movements, and use tools like the Pro Path Putting Mirror and all its features to instill/train good habits.
Find it here!