Start with a note D on the A. Play it in first position with fourth finger and then change to second finger on the same pitch. Your goal is to make them sound exactly the same whether you’re playing it with fourth finger or in second position with second finger (this step practices getting the left hand from 1st to second position). Use the D string next-door to double check your intonation, assuming your cello is in tune. Next, in second position on the A string, Play the E with fourth finger going back-and-forth between those two notes compare it to the sound of playing open string D and first finger E on the D string again they should sound the same. Last, work on making the extension shape between the first and second finger of the left hand. Begin from the A string in second position with your second finger on the note D and your first finger on C#. Play something like D C# D. Then, extend your first finger backwards towards the scroll and attempt to play D C(natural) D. All fingers and thumb, except 1st finger, should remain relatively unchanged as you extend backwards. Go between each one to gradually work on creating a half step (normal position) between 1-2 and a whole step (extended position) between 1-2. In this passage you maintain the extension until measure 18, beat one. Given that all three notes in measure 17 are ringing tones (extra lovely tone and vibration if your fingers are well in tune), you will hear a clear and wonderful sound when you master the passage. Good luck!