I play two notes at the same time and try to sing both notes. At first I started with larger intervals and now I can hear and sing major seconds. Minor seconds are still hard.

I also play one note and try to sing an interval above and below. So for example I play G and try to sing a major third above and a major third below. I then play those notes and check how close I was. I repeat a couple of times until I get it right. I do that for different notes and with different intervals.

I then play a triad and try to hear and sing the three notes. At first I could only do it for the top note, but then I was able to hear the root note, and finally after a couple of days I was able to hear and sing the middle note.