The playing starts immediately. The task sounds simple but will require some thinking and finding initially.
Put on a backing track and listen for the chord changes. Play along with the chords strumming or arpeggiating triads only. This will help you to become famiar with the structure of the progression in the backing track and the shape and location of the triads you need.
Soon, move to playing single notes.
Play one note per bar, just one note, over the chord progression within the backing track. Start with the root note of each chord as it comes around. Just the root. Get used to where the root notes are and what they sound like.
Then repeat but with the 3rd … and then the 5th.
Then repeat the above but play any two notes per bar over the chords. Combinations are root & 3rd, root & 5th, 3rd & 5th.
Try to think of variations in the way you play these notes. Remember, you are practicing as a build up to playing solos. Add a little vibrato. Slide to a note. Slide out of a note. Play hard. Play soft. Let a note sustain. Mute to kill a note.
Then repeat and extend the idea. Make up a little run of 3 or 4 notes that either start and / or end on the root note of each chord per bar. You could progress to short runs, licks & phrases. For now, still limit your choice to notes that are only from the triads – the root, 3 / b3 and 5.
Again, remember that you are building skills to playing interesting, melodic solos. What can you do to do more than just play the notes? What techniques can you bring to your short phrases? Bending? Hammer-ons and flick-offs? Vibrato? Dynamics?
Try to be conscious of ‘when’ you start your phrases too. You do not have to start everything on the count of 1.
Try to be conscious of the duration of your notes. You do not need to play just 1/4 or 1/8 notes. Can you incorporate some triplets to your phrases?
Try this for a few weeks over a variety of backing tracks in different keys. The sort of backing track that will work best with this will have only three or four chords, be fairly slow and perhaps hold each chord for two or more bars. I will post one for each example that follows. Each will be a good length and most use the famous and ubiquitous 4-chord progression I - V - vi - IV (as made infamous by Axis of Awesome).
See if you can find others, or use a looper to make up your own.