HOW TO INSTALL RESOURCE PACKS:
Download the appropriate version of pack, in this case we use WWI Pack for Minecraft 1.8.9
The the installation its same for any version.
1) Close Minecraft, open the search engine and write: %appdata% and press Enter
2) Go to .minecraft/resourcepacks/ and drag the pack inside:
3) Now launch Minecraft and go to Options/Resource Packs/ and select the pack:
Now the pack is installed in your minecraft: