Any suggestions how to slide from an open string note? One song I’m learning has a slide from open E to the octave at 12. Do I just pluck, hammer it on the first fret, and slide?
Also, does anybody have a good exercise to improve precision when sliding? I often overshoot or undershoot my target note.
Thanks, friends!
I’d just pluck and slide without hammer-on. That should be good enough.
To improve precision, I find it easiest if I set my eyes on the fret I am sliding to. The same works well if you have to jump up or down the fretboard.
What @Krescht said 
It depends a little on how long time you have for the slide… As long as you pluck the open E and then slide to land on the 12th fret (in time), it doesn’t matter a whole lot were exactly that slide starts.
The more time you have (or need to fill) with the slide, the lower I’d start; if it’s a quick slide, I’d probably start higher (to get to my target in time).
In any case, I wouldn’t worry too much about including all notes/pitches in the slide; I’d probably start in the general area of a “G” 
+1 to what @joergkutter said!
For slides, the important thing is that you do a teeny bit of slide somewhere.
If it’s an open string slide, where you do the slide (what fret you start the slide at) won’t matter.
I was going to type more, but @joergkutter said it all already!