Here are a few tips.
1. Get up in the box and crowd the plate (RH pitcher, RH hitter). This cuts the side angle down and the pitcher probably doesn't have great velocity.
2. Practice having balls come at you. Practice turning in, not out, for hit-by-pitch with wiffle balls or tennis balls. You have to keep your front side in. Many high school hitters bail.
3. If you have a pitching machine, set it to throw at an angle to the plate.
4. Pick up the ball as early as possible.
5. Relax, relax, relax
Most sidearm pitchers are successful because they are rare and the hitter is not used to the angle of attack.