No green is flat, unless your playing Putt Putt Golf at the KISS Miniature Golf. Most greens have a thing called a break. First you will want to have a pre shot routine in that you see the break you line up you take practice swings to have a feel of distance. Always remember the time of day and the location of the Sun the green is alive and changes with the hour in the day causing every shot to be different. You have to realize when putting you are putting straight and it is the break that makes that ball curve in every which way. If you simply strive for a straight put the break will take over and more and likely give you a better opportunity to sink them putts with NO GIMMIES.
1 step up shot
2 pick a dummy target
3 aim for a straight shot
4 see more and more putts sinking