First off, I messed up in the last video. VISA processing time is completely dependent on which state you're in. In Texas for example, it was only 20 days. Also, with the application itself, you need to give them an econforimation and or an itinerary, aka proof of travel. Well, if you don't want to be locked into a flight, you can purchase the airfare, give it to them for the application process, and then refund the airfare through the airline within 24 hours. MAKE SURE IT'S WITHIN 24 HOURS OR YOU'RE STUCK WITH IT. By law you can get a refund through the airline, not necessarily through a third party booking site. Be sure to check the airlines rules, but it is a good way to not get locked into a flight if you want to change anything. One thing to keep in mind, the first arrival in Brazil must take place within 90 days from the date the visa was issued. This is a good way to make sure you have some wiggle room if you want to change your plans though.
Google flights is a great tool, remember to search 10-3 weeks before you want to travel, with 5-6 weeks out being the sweet spot, however I have seen last minute flights comparable to the sweet spot. Honestly, find airfare you're comfortable with, and purchase it then. Always book directly through the airline. Third party booking sites add too many problems if things change, on either end.
With ITA Matrix, you can search a calendar to find the cheapest flights. However, you cannot book through ITA Matrix like you can with google flights, so once you've found a flight you like, take the information and plug it into the airline. Pretty easy.