Basically, just experiment with different tones.

For fandubs, some people are nice enough to upload instrumental tracks with the backings still intact so you could listen to those and try to remake them. Otherwise, I just concentrate real hard on a song where I want to add in backing vocals.

Harmonies and backings are the most difficult thing about singing, tbh, so don't worry if you don't get it right first time. It takes practice and an amazing ear.

I agree with Amy. ^_^ Listening to a track with the background vocals helps A LOT when you want to make your own.

Also, practice it before you record. Then after you're sure you can do it in your sleep, record a few takes but don't choose one right away and be done with it. Take as long as you need for it until you KNOW for sure you have finally got it down. Once you are satisfied with it you can start to piece everything else together. Putting in the practice and effort will show in your final piece. <3

I am going a bit off topic but don't just throw things together. People WILL notice. Do something that you will be happy to work on and be proud of! You don't need to rush it just because it's a HOT new song and you want to be one of the first to cover it. If you put your effort into it people will recognize you more for your hard work rather than those who rushed to do it. :3 PRACTICE PRACTICE PRACTICE! 8D

Agreed with both.

I used to have lots of troubles with harmonies and stuff, but the more I do some, the better I am at making them. After a while, you'll just be able to create harmonies of your own. ^^

What I do to practice them is that I listen to a song, just any song
and I sing over it, but I try to make it all in harmony. It's very hard, but I still do it everytime
I hear a song. After a while it just automatically comes to you.

I hope this helped. :3