Just means you're now playing the game heh. Gearing your units "perfectly" isn't an exact science, you just gotta look for the best gear you can get and put the puzzle pieces together.
So for example Karin, she
needs minimum 85% crit chance and 55% effectiveness for her kit to function. The rest of the stats depend on where and how you're using her. Is she strictly for W11 hunts? Then you can go in hard on crit damage and speed with some attack, since the Wyvern only attacks your front line tank and you want to get in big damage. If she's going to be your main Water DPS for PvE, then you gotta spread things out. You're going to want 100% crit chance when you're up against other water units, and a reasonable amount of health and defense (like 800 def and 8k health minimum). If she's also for PvP, then that raises the stakes significantly. 100% crit chance, even more effectiveness (for upper level Arena), quality speed. Nothing you're going to assemble in a day, or a week, or a month even, it's a gradual process.
But once you understand your targets, how do you put it together? Looking at crit chance first, for starters she has a base of 23% after her 4* awakening, which means you need 62% more to meet the 85% minimum. A level 70 necklace with crit chance main stat at max level adds 45%. A completed crit set (2 pieces) adds 12%. If you have that you only need an additional 5% crit chance in substats to meet your 85% minimum. Maybe you decide to go all crit set (36% total) with a crit damage mainstat necklace, and use substats to fill in the extra 26% crit chance with substats. Maybe you have some speed gear that rolled great crit chance substats and you use that. It all comes down to how your gear rolls, really.
This is my Karin. Plenty of room for improvement, but she gets the job done where I need her. Pretty key in carrying my Arena offense to Challenger too (but no further).