To make custom Red Bull livery you need to install mod from link of the top of mod description. Next thing you need to do is edit the livery in photoshop (required intel texture works plugin to edit and save). If you ends save it (you need to match the right save format for example BC3 SRGB) and call it as replaced .dds file. (The best option is opening two ego erp process with same file and try to match right save format)
View attachment 494251
I'm no longer modding and playing F1 2020 but I hope it helped you a little bit. If you still can't to do this send me on priv your discord or something like that and I better explain to you.