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:

pack_1

If you are using other operating system:
Launch Minecraft and go to: Options/Resource pack/Open resource pack folder And follow the step 2

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: