k, my two friends are feeling really ugly and fat, one of them I got to admit is fat but she has the prettiest face EVER, and the other one isn't fat at all, she's not like really skinny or anything but she is FINE, all the guys think she's a hotty.
But I just want to make them feel better about themselves, but I have no idea how

Well its easy for the first one who has a nice face u just tell her and compliment her about her face by saying flattering things about to make her feel comfortable and happy. For the second do the same tell dem they are pretty that's all man

Its never really going to matter what you say to them.
I know because I feel the same way about myself that your friends feel about themselves, however it never matters what my friends say to me because in my mind its just them trying to make me feel better with no actual meaning to it.

The best thing that makes a self-concious girl feel good about herself is having someone of the opposite sex tell them that they're attractive. That way you know that you are actually appealing to those who you want you appeal to.

Unfortunately, I doubt your position as a "friend" will have much effect on their "fretting". They're going to do it either way, mostly based on their perceptions of what OTHER people say/do towards them. Even more frustrating.

All you can do is dismiss those comments when they make them immediately and with no extra energy than is necessary... and change the subject.

HER: "I feel so ugly!"
YOU: "Yeah, well, you're not, so what do want to do tonight?"

I'm like that. I think I am ugly, but most of my friends say I am not and try to make me believe that I'm not ugly