This advice is a little old school. And yet, it's the best. You do not have to have a major idea in place. You don’t even have to have an idea in place. The creative process has nothing to do with logic, sequentiality, or reason. It kind of ambles from one topic to the next, and many novels only come together at the end. JK Rowling indicated she had no idea where the Harry Potter series was really heading. But it all tied up perfectly. Lewis Carroll's 'Alice in Wonderland' or 'Chronicles of Narnia' were free flow works. They were not sequential.
New, free, creative, and written without the audience in mind.
This is because the higher, creative mind sees the future when it transmits messages to you, now. All you have to do, literally, is commit to writing each day. Even if you are completely stymied about something to write about, write about anything. Ideas will crop up. It is law.
Go ahead and take one step, and the others will reveal themselves. This not only applies in writing, but in all exercises. Martial arts. Painting. Reading. Meditation. The biggest issue is, in fact, the logic mind and its desire to have everything laid out while it is in a lower level of consciousness. You need to bypass the logic mind, a feat which can only be conducted by simply doing something without having a goal or end in sight.
This is, by the way, an ancient philosophical key. Keep doing, the results will come. If you are eager to get results, the process is defeated. Doing without thought of results is advice often given in the Yogic lineage. Brilliance comes as a consequence. But by the time that time arrives, you will likely be detached from ‘results’ anyway.
Falling in love with process is one pillar of a happy life. So if you are a writer, be happy with a pen or computer to write with, and express yourself unreservedly.
All else, will follow.