OFFICIAL HINT: You do not have word structure to help you this time, so follow standard cipher procedure. Look at frequency analysis and then ioc (index of coincidence). This should give you a good start.
OFFICIAL HINT: Once you have figured out how long you think the Keyword is, go with it. Decode as you think it should be done, then look carefully, very carefully at your solution and see if you can find the additional twist.
OFFICIAL HINT: This is a double cipher. One cipher you have seen before lots of times. Work on that as normal. When you think (based on the ioc and frequency anaylsis) that you have decoded it, then look carefully at your solution. The second cipher should become obvious at that point.