How do I make my character skid when he is sprinting.

You will want to implement some basic velocity and acceleration physics. Rather than letting the character move at a set speed when you press a key, have a variable called velX and velY for the character. Tell your character to move velX distance left/right every tick, and velY distance up/down every tick. When you press up, increase the vely by a little every tick, and when you press down, decrease it by a little every tick. Same for left and right on velX. Then lower both velocities by about 10% every tick to get natural deacceleration. This will make him slid around, and accelerate smoothly.

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you can multiply the acceleration numbers by a speed variable to change how fast he is.

Tweak the numbers how you see fit.

If you're using the platformer

set running to when key pressed on the floor

set skidding to when is moving but not pressing the key on the floor