How to Install and Enable the Small Held Items 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 inside your game directory. Move the Small Held Items pack file (it should be a .zip archive, not an extracted folder) directly into that directory. Do not rename it and do not unzip it — Minecraft reads the archive as-is. If you extracted the contents by mistake, re-download or re-zip the folder so the pack has a valid pack.mcmeta at its root.
Enabling It In-Game
Launch Minecraft and go to Options → Resource Packs. The pack should appear in the left-hand "Available" column. Click the arrow to move it to the "Selected" column on the right. If you use other packs, drag Small Held Items above any pack that also edits item models, since the topmost pack wins conflicts. If nothing changes, confirm the pack is actually selected and not just listed, and check that no other pack is overriding the item model files.
Verifying It Works
Hold a tool, weapon, or block in your hand. The model should now render noticeably smaller than vanilla. If it looks unchanged, try toggling the pack off and on again, or restart the client.