To get started in the key of D, first tune your instrument to A-440 on the front string and a D a fifth lower on the back string. The open strings are (1) and (5) in the key of D.
Then use the ring finger of your left hand to find an octave D' on the front string. You can keep comparing it to the back string until you get a good octave. This position gives you an octave D' on the front string (1') and if you don't move your finger, you've got a G (4) on the back string.
The next finger is the middle finger, and should be placed very close to the ring finger. This is a half step. You will then have C# (7) on the front string, and F# (3) on the back string.
Many songs on the Erhu are in F pentatonic. This is a good first or second key to learn. The index finger is placed three half steps down from the qianjin string, to give an F on the back string and a C on the front string.
The second finger gives a G on the back and a D on the front. If you also use the third finger, you can get an extra A on the back, and an E on the front string. The little finger can be placed an extra half step up to give a Bb and an octave F. In this position you can easily get the notes: D, F, G, A, Bb, C, D, E, F. If you slide the index finger down a half step, you can also get the low E note.
The index finger is then placed approximately in the middle of the remaining space. Actually it will need to be placed just a little bit lower than the middle of the remaining space. This gives you E (2) and B (6).
You now have a full octave D scale. When you practice, always try to keep the lower fingers down when playing scales. For instance, to play 1,2,3 use the open string and then put the index finger down for (2). Then *keep* the index finger down and add the middle finger for (3). If you want to play four, then keep the index and middle fingers down and add the ring finger.
If you don't start the habit of keeping lower fingers down, you'll have to retrain yourself very painfully when you want to try to play more complex pieces at a fast speed. Keeping the fingers down also makes it much easier to play in tune. If you get an E in tune, and then play and F# with the next finger, you don't need to retune the E if the melody goes back down.
If you play D, E, F#, G, then all three fingers should be down while you play the G. If you then need to play an E, then lift *both* middle and ring fingers to expose the index finger. Keep the spacing the same as you lift them, the E is automatically in tune because you kept your finger down. Now if you need a G again, put both middle and ring down *together*.