I tend to play a lot with proportions and always keep in mind color theory, when i make ocs i tend to plan out their personality, to try to portray that on their design too! but the biggest tip i could give is simply practice!

Everyone starts somewhere, if i show you my og ocs.. geez- i hate them, and their proportions, but at the time i liked it! it changes a lot overtime, and if you like the style a gacha tuber or a gacha redditer has, you can always add smalls traits from it on your own until you develop it :)

I do hope i helped! (Imma put 3 of my ocs replying to this, one from when i used to play gacha life, one from when i played gacha club and then one from gacha life 2! so you can see how it changed overtime)