@ Toby … that's right, simple to start.

Before spending time on these practice exercises, you can prepare a little with some warm-ups that will benefit in a specific and general sense too.
If you're going to play, for example, over a backing track in D (triads D, A, Bm, G).
a] Find and play all the root notes of the I, IV, V and vi chords on all 6 strings using the '1st-finger only technique' as taught by Justin. You will be learning to find the notes all over the neck. This will set you up to be able to more easily locate the root notes of the triads when targeting chord tones.
b] Play the D Major scale up and down using alternate picking in each of the five CAGED positions. This will give you an immediate connection between the triads you're going to play, the other D Major scale notes that link them together and an overview of the key you're playing in.
c] Play the triads through several repetitions of a short progression, as mini-chords either strummed or arpeggiated, changing the position and fingering freely at will ranging across all parts of the neck. This will help familiarise yourself with the entire triad shape as a whole before you begin targeting and playing individual notes from them.
Additional ideas.
Start each session of targeting the chord tones over your backing track of choice by going back to the very first exercise of playing the root note only over each chord when it comes along. Use this to reinforce the root note sound and positions. Also use it to practice your single note vibrato technique.
When playing short runs and phrases around the target notes, try to develop a motif for the tonic chord (of say a handful of notes including the target note root, 3rd or 5th) and repeat it, or small variations of it, on the other chords. In this way you will be starting to play lead lines that have connecting themes and ideas that are telling a story and connecting back to each other.
For example:
R * * R * 3rd
where * are other notes drawn from the Major scale not in the triads.