Playing slide in regular tuning is more difficult than when your guitar is re-tuned to an “Open Tuning”.
Here is a demo of how I play in standard tuning – showing (I hope) clearly how I dampen the other strings that I don’t want to ring out.
Basically, I pluck the intended string with my index finger – AND use the side of my thumb (and the tip) to dampen the bass strings above the string I want to play, and my other fingers to dampen the strings more treble to the string I want to play.
With a bit practice it becomes easy – even though it might look too complicated at first.
I hope this helps someone !