How would I say a person singing is good.
 

If I wanted to say her singing is really good！

how would I say that

彼女の歌う本当にすごいだよ。そして彼女は可愛いだよ ！

That sentence isn't correct - a person's singing voice is 歌声 (utagoe)

I probably would say

彼女の歌声は(とても)綺麗 > Kanojyo no utagoe wa (totemo) kirei > Her singing voice is (very) beautiful

I'd get a second opinion though lol

We Japanese never place だよ drectly after an adjective. If you do you will sound VERY foreign. Directly after a noun, you can.

Use んだよ if you want to use a form of だよ.

Example: すごいんだよ。可愛いんだよ。高いんだよ。うまいんだ よ。

But just in case you are still unfamiliar with the use the more formal ですよ form, don't even bother with this colloquial んだよ form. Just a native speaker's opinion.... I'd get pretty irritated if a stranger much younger than me came up to me and used the んだよ form.

この子の声はすごく優しいで、美しいで.........

Just my opinion. May not be right lol XD
You want to describe her voice using adjective other than good?

Nice try, but we don't put で right after an adjective.

Awww, 　is It 優しくで、美しくで? not sure lol

te is used with a -i adjective and most verbs.
優しくて
美しくて
可愛くて
de is used with a -na adjective, a noun and mu/nu/bu-ending verbs.
美人で
綺麗で
好んで

Ah, I always mix up the de and te, haha , Ty