you may already be performing correctly, but let's get back to basics...
if you are right handed, the cutter goes in your right hand and your left hand holds the ruler down. if you are left handed, go vice versa, but your rotary cutter blade needs to be changed to the other side.
while you are learning, cut through the least amount of layers of fabric - from front to back...not toward you, but away. sometimes, the fabric shifts.
to practice cutting fabric without a "v" forming, fold over one piece of fabric to form a double layer. don't use the bottom of the ruler as a starting point. instead, start at another point, let's say the half-inch mark. match the mark that appears on both edges of the ruler.
another point: to start with a straight cutting edge, carefully cut to the right of the fabric. when that is square, flip the fabric over and adjust the fabric so that the layers are straight. using your ruler, measure the fabric from left to right and cut it using your rotary cutter.
another point: while you are learning, you might not want to cut long lengths of fabric. this might be pulling your fabric. vice versa for left handed.
when you make sure that the basics are addressed, then you may have different questions.