Practise the perfect intervals with open strings (that is the 1st, 4th and 5th degrees of the scale, which in A major are the A's, D's and E's).
Change bow as often as necessary and play until the note is exactly in tune and resonant. To begin with take as long as needed on each note, but gradually work towards adjusting quickly.
Also work with each note in context. Practise playing each note that you have tuned following after the previous note so the correct interval is learnt.
Once you are satisfied with the intonation, you need to repeat the passage in rhythm, both slowly and at tempo.
Introduction to Practice - for Tempo
- Tuning A Major Scale to Open Strings - violin
- Tuning D Major Scale to Open Strings - viola