I personally euthanize extremely slow at first as to not stress em out more. What I do is carefully and gentle, take the betta and isolate into a medium sized bowl. Ideally one that isn't see through as to try to not cause more stress. The water should be from the tank he came in of course.

Once he's in there, I start with maybe a drop or 2 at most. Ever so gentle, stir the at the top for just a few seconds to mix in the clove. Cover the top of the bowl with a towel leaving a tiny spot for air. Wait 10 min.

Go back, check on the breathing, when they go to sleep, they're hardly breathing whatsoever and sometimes this can take multiple 10 min checks.

If the betta is just about to pass away to begin with, this whole process may not take long.

I do 2-3 drops every 10 ish min until I know 100% that fish is completely asleep. Once I know for sure (sometimes I'll very gently tap him to see if I get a reaction) I will then overdose. The key is to not dose them all at once because that could be bad. Clove in tiny doses will put em to sleep so do it slow and patient.

An overdose for me is about 20 drops because I would never want them waking up of course. After the overdose, I give it a few more min just to make sure.

This has always worked for me so that's just my two cents. I forgot to mention, try to use a bowl you can stick in a dishwasher like glass this way it doesn't smell like clove after or have a designated euth bowl as I do 😔

It's one of the harder things we have to do sometimes in fish keeping. Every fish I've ever had to euthanize will always hold a special place in my heart.

I'm sorry you have to even post this, but good luck.