How to Install and Enable the Modern Armour Bar Resource Pack
Placing the Pack in the Correct Folder
A resource pack only works if Minecraft can find it. Open your Minecraft installation folder and locate the resourcepacks directory. Drop the Modern Armour Bar pack file (the .zip you downloaded) directly into that folder without unzipping it. If you extracted the archive, Minecraft will not detect it, so keep it as a single compressed file.
Activating It In-Game
Launch Minecraft and go to Options → Resource Packs. You will see the pack listed under "Available." Click the arrow to move it to "Selected." The order matters if you run other HUD packs: place Modern Armour Bar *above* any pack that also edits the armor icons, otherwise the other pack will override it. Click Done and the armor bar should refresh immediately.
Confirming It Loaded
Enter a world and take damage while wearing armor. The bar above your health should now show the slimmer, minimalist icons instead of the vanilla chestplate shapes. If nothing changed, check that the pack is compatible with your Minecraft version and that no other selected pack is overriding the armor textures. Toggling the pack off and on again usually resolves a stale cache.