How fuckup-proof is replacing your own iPhone battery?

Generally iPhones are difficult to self repair. You can check ifixit for your specific model and see if the directions are doable for you. You’ll generally need some esoteric tools like the pentalobe screw bit.

I worked at an Apple Store for many years as a genius during the iPhone 4 to X era and I can tell you that with all the tools and experience, about 2-4% of battery repairs would have some kind of issue that would need further effort to address caused by the repair. Some were minor, like having to replace a vibration motor or a screen, some were major where we would just replace the device.

Without tools and experience, I would imagine the percentage chance of issues would be higher, maybe 10-15% chance. From there it depends how "big" a problem that would be for you if not everything was working well after your repair.

I think 85-90% chance of success is not too bad but I agree with the other poster that this is not an easy repair