Plotting everything in detail in advance is a bit too much, perhaps divide it into stages, plan each one in more detail as the previous one winds up? Doing it as the month goes on feels better. o.O
What are the phases? Early Years, Middle(~2-5), Late (6-7), and then the War. The last one is going to be the most complicated... so let's ignore it for now.
2. Major events: James, Sirius, Peter figure out that Remus is a werewolf, Animagi research, Severus' Slytherin friends get darker, Remus and Lily are prefects, Animagi achieved, runs, Severus develops his theory, Severus and Lily start fighting, Sirius tricks Severus, Snape's Worst Memory
3. Spoilers! Yeah, I'm going to be that obnoxious. I've got the notes I need for that on paper, though.
Doing this in more detail will get a little repetitive, continuing on paper.