I am relatively new to the game and only played it casually, but im interested in trying out speed running it casually, and i haven't found anything yet about how to buttslide, sorry if i missed something obvious hehe

When going down a slope, hold down to do a slide and let go of forwards. Then, as soon as you've picked up enough speed, just jump. As long as you're not holing forwards the game will not cap your speed and you can conserve it through well timed jumps. Jumping is optimal as if you keep sliding on the ground you will lose speed.