Discs often want to turn naturally, especially at lower speeds, so getting a disc to fly straight needs a flat release angle and more arm speed.
Stand straight up with your throwing arm facing the target. Pull the disc across your chest, following through with a backhand slapping motion.
Throw the disc like a ball, leading with the elbow. Lean to the side about 15 degrees and release the disc flat while you spin it with your wrist.