How do I script a block so that players slide on it??

There's actually no scripting needed here! Under any Part's properties, there's a category called Part. In there should be the option called Custom Physical Properties. Activate this option by checking the box and click the arrow that appears next to it. This will be show a drop-down menu with many properties. The one you're looking for is Friction. Increase this or decrease this to your liking.

You can also if you are not familiar to physics, you can change the part material to "Ice" It works as a substitute, but I'd recommend changing physic properties.