A drill I like to use is a Dave Pelz drill not sure of the name but I call it the 20 ft drill, maybe he did too, not sure.
I start with a fairly level, straight putt and mark off 7 paces on opposite sides of the hole. I try to get the ball past the hole but no farther past the bottom of my putter grip. (inside the leather). When I do this 10x in a row I move to the opposite side. If a putt is short of the hole or outside the leather the count starts over. This drill really helps judging speed.
Another one I use is my 3,6,9,12 drill
Again fairly straight flat putt. Using 5 balls.
3 ft make 5 in a row move to 6ft
6 ft make 5 in a row move to 9ft. Miss 1 start back at 3 ft
9 ft make 4 of 5 move to 12 ft. Miss 2 go back to 6 ft.
12 ft make 4 of 5. Done. Miss 2 go back to 9 ft.
I used to start from the beggining if I missed the number at any distance but that got really frustrating and did more harm then good.
I have a number of winter drills i do but know my indoor range has a "practice green so i just do the ones above.
I far as on course putting goes what really helps me is taking 3 or 4 practices strokes looking at the hole then my minds has a fell for the distance. Seems to work.
The around the world drill is fun, especially with a partner for some "skins"