Watch the Legato Slide online guitar lesson by Chris Buono from 60 Electric Guitar Techniques You MUST Know
The most basic of sliding techniques, the legato slide, calls for the initial note to be picked and the destination note to simply sound as result of the slide. Put simply, don't pick the second note!
Looking for inspiration? Check out Guthrie Govan's "Waves" or Eric Johnson's "Cliffs of Dover" for sliding bliss by way of gliss. See what I did there?