make sure the fingers are not too fat and the fingertips are round in manga(even realistic manga) fingernails are not so defined.
another thing, when you do hands, look at yours, and see which fingers are the longest. it goes in this order(highest-lowest) Middle>Ring>Pointer>Pinkie>Thumb.
in the upper right specifically, dont draw lines so large to show knuckles. draw smaller ones that dont go all the way around and slightly angle to hand, whick is really never completely flat. on the knuckles where the fingers begin, make sure that they're round. last thing, the hand narrows downas it gets closer to the wrist.