Windbound

For this achievement you have to fill your entire available inventory space. So if you have a bag with 18 additional spaces, you would also have to fill those spaces. Also, each space must be completely full. Having a single flower in each space doesn’t work. This means that the minimum amount of flowers needed for this is 140 (20 per slot in held inventory)
Couple tips to assist:
Find an island that has a lot of flowers. In chapter three, the starting island always had 12-15 of both Fluttercup and Sealily for me. They generally replenish every in game day, so you can just grind it out.
If you have a bag equipped, the game will only allow you to exchange it for another bag, you can’t just drop or delete your equipped bag. To get around this, build a bag rack for your boat and transfer your bag to it. If you already maxed out your bag racks with bags, deconstruct the raft platform containing the rack. The bag rack will also deconstruct and the bag will separate but remain with all of your items still inside (so do this next to land obviously). Now you can simply reconstruct the raft base, put a new bag rack on it, then hang your current bag.
Leave the other bag on the ground until you finish the achievement, then you can go pick it back up. In my experience things on the ground tend to stay there, but I would recommend finishing the achievement before saving and quitting just in case so you don’t lose your stuff.