Just go online to the evisa website and start the process. You can save it as a draft and come back later. You will see that they ask for travel dates, port of arrival, inbound flight number, and accommodation in Thailand, but don't require any documentation for it on the application. Of course they may ask for that as a follow up via email, but they don't on the online application.
Based on all that, I would say that ideally you should have your flight already booked but don't necessarily need to have your accommodation booked. Just put down where you think you might be staying. If they follow up with an email asking for a booking confirmation, then go on agoda and make a cancellable reservation for two nights. I haven't heard of anyone being asked for that additional documentation but it's possible. I have done that exact thing before when applying for a tourist visa after they wouldn't accept my PDF of an email confirmation at a condo but with the DTV they don't ask for that documentation in the first place.