One of the most important parts of applying to graduate school is identifying why you want to apply. What areas of mathematics interest you? Why go on to grad school instead of applying for a job elsewhere?
Since I was applying for the NSF GRFP, which was due in mid-October, I prepared my GRFP application first, then modified it for my graduate school applications. This was extraordinarily helpful for me, since it meant that the bulk of my graduate school applications were essentially finished by mid-October.
I found forums such as GradCafe very unhelpful in many respects, including actually putting together my applications and my mental health.
Timing and identifying programs
In August before my senior year, I put together a spreadsheet with all the programs I was interested in, application fees, deadlines, links to the program website and application portal, whether or not the program required the GRE (general or subject), how many recommendation letters were required, and the names of 3-5 faculty I was interested in working for. Looking for faculty in this way helped narrow down my list.