Singing with your diaphram means that the voice needs to come from the stomach instead of coming from the throat. Singing with your throat hurts a lot. You can test to see if you are actually singing with your diaphram if you put your hand on your stomach while you sing. You should feel it inflating.

Singing from your diaphragm takes practise but it is the most important thing you will learn to do as a singer.
Stand up straight and place your hands just below where your ribcage ends. Now, breathe in slowly, imagine as if the breath is filling every part of your body up slowly from your feet. You will feel your stomach expanding, this is good, its how you should always breathe. Even when you are snatching a breath during a song, still make sure you push it out as far as it will go. Now, sing a note on "la" and as you sing, pull in your stomach. Try it with a higher note and pull your stomach in harder for this one. When you sing you must always pull in your stomach, its how you sing from your diaphragm. Now, try a song with a few sudden high or low notes. Keep your stomach taught for the whole time (remembering to push it out for breaths in) but pull it in suddenly hard for sudden high or low notes. You need to pull it in so hard it hurts for really high notes. This is how you should sing every song from now. It may feel a little strange at first but if you practise enough you will do it without thinking. Your throat is hurting because you are working it too hard. You are damaging it every time you sing so that it hurts. You need to give your voice a rest for about a week and drink lots of water. You should be on a litre a day. You need to take your singing slowly and hope you haven't damaged your voice permanently. If your voice starts to hurt again then stop for that day and the next day before starting again. Always make sure you have warmed-up for atleast ten minutes and you feel thoroughly warmed-up before starting singing, you need to take care of your voice.
Good luck with your singing, hope this helped 🙂

Singing harmony is just like singing the melody of any song, except that you're not singing the melody itself. You would practice singing the harmony music line by rehearsing it by yourself, note by note until you have learned the music completely. You can then sing the harmony music line, while your brothers sing their own music lines.