I'm new to this type of modding, how does the account linking work?
Do not use it on your main account. The mod detection is very active. The idea is to start a dummy account (on emulator or a spare device) and carry your main account.
If you feel modding/grinding for potions and jewels (i always lack small jewels...) via regular co-op is enough, start a dummy account using <for account linking> apk, complete tutorial and story part 1 legit, then install/update the <mod menu> apk over the <linking> apk. Turn on permissions and you are ready to grind regular co-op. If you get banned, rinse and repeat with a fresh dummy account.
If you just want to know to data link, install <linking> apk. At the title screen, after choosing Global or JP, click data link on the bottom right. If the screen brings you to Google Play, tap Cancel or hit back on your device. It will say "Verification Failed" and you can then proceed to sign in via Klab ID. Same thing for if you wanna data link a dummy account to a Klab ID account after you have started gameplay, it can only be done on the <linking> apk. The <linking> and <mod> apks are meant to be patch-able over one another, but they can NEVER be used to patch over a legit playstore version. So, if you need to switch between accounts using the mod, just patch the <linking> apk over the <mod>, change accounts then patch back the <mod> apk with permissions.
I repeat: at the moment, it is not safe to be used on a main account.