Aiming correctly on the putting greens is the first step to improving your line and accuracy. All the pros on tour line up their ball while bending down behind the ball and looking down their line.
Once you've read your putt and made a decision on how the ball will break, then you are ready to line up the ball. Place the ball down so the lettering or logo on the ball is facing the line you want your ball to start rolling on. Then when you set up the putter behind the ball, match the line on the ball with the line on your putter.
Once you've aligned the putterhead correctly, then get your feet, hips, and shoulders parallel to the line you want to start the putt rolling on. If your feet or shoulders aren't parallel left with the target line of the putt, you can move the putter off line and push or pull putts.
It's also important to have your forearms lined up parallel with the rest of the body. Sometimes a golfer who has a conventional putting grip will get their right arm a little ahead of the left. A golfer who putts cross handed might get the left arm ahead of the right. It's important to correct this if it is an issue.
When I learned this tip about getting the forearms parallel it made a huge difference in my putting and has also helped many of my students improve their line when on the putting greens.