How good of a snowboarder do I have to be if I want to be an instructor?

You should learn it properly at first.

I learned to snowboard and 1 year later started as a volunteer instructor for children - they showed me how to instruct.

If you want to be an instructor at a commercial resort you will probably want to get an instructor certification - like AASI Certification

Any hasbeen could do it.

You should be comfortable riding all terrain without stress at the mountain you are applying to instruct at. You should also make sure that you're comfy in all normal weather/snow conditions- that you are okay riding when it's snowing or getting dark, when it's windy, when it's very cold or quite warm, in powder, on mostly ice, on slush, and on terrain that has been carved up by a busy day and is starting to freeze.

If not much phases you, then you should be just fine!

It depends what qualifications the resort required. Small resorts that don’t have a lot of vertical typically have lower requirements for instructors however they’ll have you do a bunch of on snow training and determine if you’re capable of teaching others. There are definitely instructors that are more capable than others that would typically handle more advanced lessons. The best thing to do would be to call the resort and talk to the ski school director. That person would be able to tell you exactly what’s required.

Usually you can get a certificate or something that says you are qualified to teach others.