First you should download the mod from the owner of the mod :
https://www.gtainside.com/en/gta5/cars/135036-porsche-911-gt3-997-old-amp-new-add-on-template/

Then Install My livery designs with OPENIV :

Mods>update>X64>dlcpacks>oldnew>dlc.rpf>x64>oldnew_vehicles.rpf>oldnew.ytd

Follow this info and then open edit mode

After that open oldnew.ytd click import button and choose my liveries.