Just practice exercises, ensuring your thumb is lined up behind your middle finger.
On any string, any fret
Index 1, middle 2, ring 3, pinky 4
Play
1,2,3,4; 1,2,4,3; 1,3,2,4;
1,3,4,2; 1,4,2,3; 1,4,3,2
2,1,3,4; 2,1,4,3; 2,3,1,4
2,3,4,1; 2,4,1,3; 2,4,3,1
3,1,2,4; 3,1,4,2; 3,2,1,4
3,2,4,1...