All examples below are using a G chord. Use a metronome. Focus on thumb independence. These exercises focus on pinches with thumb and middle finger. I suggest you also try them with thumb and ring finger.
m m m m
p p p p p p p p p p p p p p p p
E------------------------------------------------------------
B----3-------------3-------------3-------------3-------------
G------------------------------------------------------------
D------------------------------------------------------------
A------------------------------------------------------------
E----3--3--3--3----3--3--3--3----3--3--3--3----3--3--3--3----
1 2 3 4 1 2 3 4 1 2 3 4 1 2 3 4
m m m m
p p p p p p p p p p p p p p p p
E------------------------------------------------------------
B-------3-------------3-------------3-------------3----------
G------------------------------------------------------------
D------------------------------------------------------------
A------------------------------------------------------------
E----3--3--3--3----3--3--3--3----3--3--3--3----3--3--3--3----
1 2 3 4 1 2 3 4 1 2 3 4 1 2 3 4
m m m m
p p p p p p p p p p p p p p p p
E------------------------------------------------------------
B----------3-------------3-------------3-------------3-------
G------------------------------------------------------------
D------------------------------------------------------------
A------------------------------------------------------------
E----3--3--3--3----3--3--3--3----3--3--3--3----3--3--3--3----
1 2 3 4 1 2 3 4 1 2 3 4 1 2 3 4
m m m m
p p p p p p p p p p p p p p p p
E------------------------------------------------------------
B-------------3-------------3-------------3-------------3----
G------------------------------------------------------------
D------------------------------------------------------------
A------------------------------------------------------------
E----3--3--3--3----3--3--3--3----3--3--3--3----3--3--3--3----
1 2 3 4 1 2 3 4 1 2 3 4 1 2 3 4
m m m m
p p p p p p p p p p p p p p p p
E------------------------------------------------------------
B----3-------------3-------------3-------------3-------------
G------------------------------------------------------------
D------------------------------------------------------------
A-------2-----2-------2-----2-------2-----2-------2-----2----
E----3-----3-------3-----3-------3-----3-------3-----3-------
1 2 3 4 1 2 3 4 1 2 3 4 1 2 3 4
m m m m
p p p p p p p p p p p p p p p p
E------------------------------------------------------------
B-------3-------------3-------------3-------------3----------
G------------------------------------------------------------
D------------------------------------------------------------
A-------2-----2-------2-----2-------2-----2-------2-----2----
E----3-----3-------3-----3-------3-----3-------3-----3-------
1 2 3 4 1 2 3 4 1 2 3 4 1 2 3 4
m m m m
p p p p p p p p p p p p p p p p
E------------------------------------------------------------
B----------3-------------3-------------3-------------3-------
G------------------------------------------------------------
D------------------------------------------------------------
A-------2-----2-------2-----2-------2-----2-------2-----2----
E----3-----3-------3-----3-------3-----3-------3-----3-------
1 2 3 4 1 2 3 4 1 2 3 4 1 2 3 4
m m m m
p p p p p p p p p p p p p p p p
E------------------------------------------------------------
B-------------3-------------3-------------3-------------3----
G------------------------------------------------------------
D------------------------------------------------------------
A-------2-----2-------2-----2-------2-----2-------2-----2----
E----3-----3-------3-----3-------3-----3-------3-----3-------
1 2 3 4 1 2 3 4 1 2 3 4 1 2 3 4
m m m m m m m m
p p p p p p p p p p p p p p p p
E------------------------------------------------------------
B----3-----3-------3-----3-------3-----3-------3-----3-------
G------------------------------------------------------------
D------------------------------------------------------------
A-------2-----2-------2-----2-------2-----2-------2-----2----
E----3-----3-------3-----3-------3-----3-------3-----3-------
1 2 3 4 1 2 3 4 1 2 3 4 1 2 3 4