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; 3,4,1,2; 3,4,2,1
4,1,2,3; 4,1,3,2; 4,2,1,3
4,2,3,1; 4,3,1,2; 4,3,2,1

With index on 5th, middle on 6th, ring on 7th & pinky on 8th fret play the following pattern on each string...
5,7 low e string to high e
8,6 high e to low e
7,8 low e to high e
7,6 high e to low e

Finger stretch - you may need to start on 7th or 9th to begin with...
Index on 5th middle on 7th low e to high e and back
Middle on 6th ring on 8th low e to height e & back
Ring on 7th, index on 8th low to high & back

Final power chord index 7th low e, middle 9th a string. Play shuffle feel (bom bom....bom bom) on the forth play with your pinky on the 11th fret of the a string.

That should help you build some dexterity. Use a metronome!