How to transfer my Xbox 360 minecraft worlds to Xbox 1?

Tomorrow I'm trading in my Xbox 360 and I want to transfer my minecraft worlds to my Xbox 1 that I will get tomorrow. I know you can upload one world at a time but I won't have my Xbox 360 and Xbox 1 at the same time so I won't be able to do that. I have my worlds saved to a flash drive but I don't think that will work when I stick the flash drive into my Xbox 1.

When you go on mc xb1 there's an option to transfer worlds. But it did not work for me from ps3/4 it just said it couldent do it. I don't know why.

You don't need them both on at the same time. You upload a world from the 360 to their servers, and then later on the One you can download it. It doesn't go straight from one console to the other.