Stabbing/cutting is not as effective as you think it is -
https://sanctioned-suicide.net/threads/non-methods-that-should-not-be-attempted.177799/unread
If you still want to go through with this method and you've made up your mind already i would recommend using a pocket knife, the ideal place would be through the neck, target the arteries or blood vessels, theorically, if this works, your body won't be able to transfer oxygen blood to your brain and you will die as a result of that
Here's this video which might give you a general idea of how you can do it
https://watchpeopledie.tv/h/social/post/201619/man-accidentally-stabs-himself-with-not (this is accidental, but you get the general idea)
Keep in mind this is not very effective and i would advice against cutting or using a knife, you can end up disabled if you survive a pocket knife stab in your neck, the chances for surviving is very high and ultimately it's not worth the risk compared to other safer methods, you can look through
https://sanctioned-suicide.net/threads/suicide-resource-compilation.3/ to see what suits you best