You need to unzip the file then go to your C Drive, Program Files (x86), Steam, steamapps, common, F1 2019, 2019_asset_groups, f1_2019_vehicle_package, teams, wep.
Then you need to open the ERP file, click XML files on the lower row of tabs. Choose the team you chose (unless it's Mercedes in that case you need to add the file to every team including the FOM Car file because there is a Merc file in every team for some reason). After you click the team you want, click the XML files tab at the top and choose the file you downloaded. Then click save and it should be good to go! Let me know if you have any other questions :)
Then you need to open the ERP file, click XML files on the lower row of tabs. Choose the team you chose (unless it's Mercedes in that case you need to add the file to every team including the FOM Car file because there is a Merc file in every team for some reason). After you click the team you want, click the XML files tab at the top and choose the file you downloaded. Then click save and it should be good to go! Let me know if you have any other questions :)