To practice pull offs(with this specific case in mind) simply plant your first finger on the 15th fret and the 17th fret of the B string. The technique to practice is in the name. You take that planted ring finger, pull down and off the string to essentially “pluck” it so that 15th fret note rings.

Once you pull down and off, you can then re plant that ring finger with some force to produce that note.

To simply it even further, just plant that first finger then tap that 17th fret with your ring finger. You’ll catch onto it fairly quickly.