The Hanon finger exercises have been a staple of piano teaching for more than a century - and with good reason. They are easy to understand and therefore to play and they train the fingers to obey mental commands as well as warming the hands up nicely for repertoire practice and performance.
Anybody hoping to become or remain a proficient keyboard player or performer needs technical practice to maintain any level of skill. Supplemented with more advanced technical exercises as necessary, the Hanon finger exercises offer a simple first-choice solution for doing just that.
Here's Hanon Exercise No.15, played straight.
Now go on to Exercises 16 to 20.