Many bottleneck tutorials will make demands about the gauge of string you use or the action of the strings on the neck or any number of other things. What they're all generally getting at (amongst other things, to be fair) is that it all comes down to pressure.
The idea behind using the slide or bottleneck is to have it stop the strings from vibrating at a given point along the string's length, much in the same way that the frets do. Untouched, the string will vibrate the length of the string. By placing the slide against the string(s), you stop it vibrating to change its pitch.
So how much pressure is the right amount? To an extent, that's between you and your instrument. Too little and the string won't stop the vibration cleanly and it'll rattle and zip and buzz. Is that a bad thing? Not necessarily. Too much pressure and you'll rattle up and down the frets like Big Joe Williams to name one. In general, you'll be aiming for something in the middle.
It's well worth spending some time practising and experimenting with the different sounds you can make with different amounts of pressure when using the slide for long-or-short slides or fast-or-slow slides.
The string gauge and action of your guitar will help you with the pressure. A heavier gauge of string and higher action gives you more margin for perfection. Higher tension means that the strings will fight the slide back more and lets you dig in a little harder.