1) Start by downloading the appropriate version of  pack, in this case we use WWI Pack for Minecraft 1.8.
The the installation its same for any version. Launch Minecraft and click on Options:

2) Click on Resource Packs…:

3) Click on Open Resource Pack Folder:

4) Drag the Pack file into the resource pack folder:

5) Now refresh the packs selector, click on Done to go back:

6) Click on Resource Pack… again:

7) The pack now appears, select the Pack from the left column and move it to the top of the Selected Resource Packs column

8) Select Done the game will now start to load textures 

8) Now the pack is installed in your minecraft: