Ziploc Big Bags
Plastic Storage Bin
You DO NOT need - tape of any kind, cardboard, paper or glue of any kind.
Throw away or clean anything with even a small stain on it. This will really help you weed through what you are keeping. Even a small stain can get all over other clothes in extreme weather, you wouldn't believe how many things can get ruined by a small stain on a sleeper that you missed so be careful. I am so glad I checked Ava's newborn clothes after a year and figured this out early.
Lay the clothes in flat stacks and seal them in the zip lock bags. Label the outsides of the bags by size with your sharpie. Lay the bags in the bins. Label the outside of the bin with the Sharpie. Do not tape the bin or label with tape or stickers. Glue and cardboard attract pests or at least in our Florida attics and garages they do.
If you are storing shoes put half of a dryer sheet in each shoe and put them in individual sealed Ziplocs first. This will keep a pair of smelly shoes from infecting all of your shoes.
Finally, even if your child outgrows a size in stage just keep them together in their closet (like on a shelf to the side) until you are ready to pack and clean out an entire size because if not you will be sorting! Sorting stuff you might never use again is not my idea of fun. You can slip the large bags under a bed until you have enough to fill a bin for the attic.