If you don’t turn your shoulders horizontally, you will likely hit the ball into the net.
If you don’t turn your shoulders enough vertically, it is difficult to get lift-off from the ground, and you will likely be hitting the ball into the net.
If you drop your left hand too soon, the ball will fly horizontally with no spin or slice, and likely hit the net.
If you pull your head down and take your eye off the ball before contact, it will cause the ball to be pulled into the net.
If your toss is too far left, you’ll likely hit the ball into the net because you cannot hit under the ball.
If your toss is too far to the right, it will also be hard to hit under the ball, and you will likely only be able to get the ball in if you hit it with slice. It will feel like all arm.
If your toss is consistently errant, it may be because you are shifting your weight between your feet at the wrong time, causing the ball to move with your body weight. Your body should be still with your weight on the front foot, moving slightly forward when you toss the ball to encourage a slightly forward ball toss. Errant ball tosses can also be caused by a bent left arm, or moving shoulders too much during the toss. I find it is helpful to pause as I rock back, and breathe out slowly to relax myself. Being relaxed helps the toss be more consistent.
If you are hitting many balls into the net consistently, check the above problems. If that does not work, consider checking your overall rhythm. Your motion should consist of an easy toss, body coil, and hit. You should be able to count “one, two, three” with plenty of time as you practice your rhythm.
If you can’t get much spin, keep the face closed on takeback (salute style), with your palm down, rather than opening the strings up to the sky.