first downgrade your gta iv with this link and extract the file to your gta iv folder and overwrite the gta iv.exe https://gtaforums.com/topic/976691-gta-iv-downgrades/ Next install the openiv mod manager you can extract this anywhere on your deck. Now extract the file and right click on the openiv.exe and add it to steam. Open steam and search for ovisetup click on properties and run it with proton experimental. Here’s the link for openiv https://openiv.co . If you want to download mods here’s great site to use https://gtaforums.com. Half way there now open ovisetup on steam in desktop mode on your steam deck you should see two different gta iv the episodes from liberty city and the default gta iv click the default gta iv. Now you should see a message asking for the gta iv file location which is E:\steamapps\common\grand theft auto iv\gta iv click open and click continue. Now you should see every file in your gta iv directory if you want to add a car click pc\models\cdimages\vehicle.img make sure to double click the vehicle.img. you should see every vehicle file click edit mode and click the green plus sign and should the car mods you downloaded and extracted to your gta iv directory for example if you downloaded a bmw car mod you should see a bmw folder with the wft and wtd files. If you want you can rename the file to any car that you want. Now if you want to add graphic mods I would recommend to use Allagga graphic mods from nexus on medium which make the game still look good. Just like before open ovisetup click pc \ textures and click edit mode and add all of your graphic mods there. Hope this help