See a PGA instructor and practice what you learn from them. =)

The video on how to putt better with Brad Faxon is a great place to start. Check out the youtube videos from Titleist. They have some good tips.

Practice putts from 5-10-15-20 feet and beyond.
I always have a method for calculating distance and pace.
Let's say I have a 20 footer uphill, this is kinda my process:

1. calculate the distance.
2. calculate the slope.
3. zero in on start line.
4. add pace for the up slope.
5. All Systems Go!!!

Most importantly is to develop a process that is fluid. Keep things moving and don't over do it.

Cheers,
Dr. K

It depends on where you're struggling. If you mean you're having problems with lag putting, the drill I've used (from one of the Titleist staff instructors), was to try to putt into a 4 foot box around the hole, starting from about 40 feet away. Any putt that ends up in the box leaves you with a 2 foot putt to the hole. Fairly easy two putt. I do this when I'm on the practice green before a round to get a feel for the greens. Do this from multiple angles will greatly improve your distance.

I would find a PGA instructor in your area. This would help a lot more than going down the Youtube rabbit hole.

Go to your local course and ask the PGA pro for a lesson. Practice what they preach.