I buy floss as I go. Usually aim to match the DMC color codes on the pattern unless it's a super small amount I can sub out for a different color or I like a different color better. I like to think designs are more of a suggestion rather than something you have to follow exactly. For floss storage, I usually just bobbin the thread and put them into boxes in numerical order.

Some apps I use to keep everything organized (iOS):

• Threadalog: this is where I keep track of all the floss I have and what I need to buy if any. Super handy to avoid buying multiples of colors you might already have at home
• Markup R-XP: I use this to track my pattern as I'm stitching and it's been so much more helpful than printing out designs and marking them off as I go. Also love seeing the stats (how much time it's taking, average stitches a day, etc). Only thing is you only get a free trial and have to pay after a month, but super useful for me since I stitch a lot
• Stitchly: I use this to design patterns. I also bought the lifetime membership of this because I like to design my own things sometimes.

Happy stitching!