How to Install and Enable Better Bubbles in Your Resource Pack List
Placing the Pack in the Correct Folder
Better Bubbles is a resource pack, not a mod, so it does not go in your mods folder. Locate your Minecraft resource packs directory, which is the resourcepacks folder inside your game directory. Drop the Better Bubbles pack file (or its unzipped folder) directly into that directory. If Minecraft is already running, you do not need to restart the game for it to appear in the list.
Activating the Pack In-Game
Open Options, then Resource Packs. Better Bubbles should appear in the left-hand column under Available. Click it to move it to Selected. Order matters: if you run other packs that also touch the HUD, place Better Bubbles above or below them depending on which bubble style you want to win. Packs higher in the list override lower ones. Click Done to apply.
Confirming It Works
Enter any world and go underwater. Watch the air bubbles above your hotbar. If the style changed to the smoother, cleaner design, the pack is active. If nothing changed, confirm the pack is in the Selected column and that no higher pack is overriding the bubble texture. You can toggle it off at any time the same way.