Practice
Again focus to start with on learning these new skills, one at a time. If picking with your fingers is not yet feeling natural you won't be able to fully foucus on getting your pitch right with your slide so you may want to pick this one with your thumb on the bass string at first. Take some time to get both hands used to the new skills before combining them.
When these skills feel more comfortable you're ready progress on to the song. Try to structure your practice sessions to give all the technical exercises, the song and the free form playing (we'll call it improvising from now on) equal time.
From the ebooks
Again from Exercises for Left and Right Hand.
- Studies no 2 and 3 for slide guitar from page 27
- Studies no 2 and 3 for fingerstyle guitar from page 44