Hey man, sorry for the real late bump, but here’s how you would go about doing this. Export the meshes of the actual part you want liveries applied to from Roblox. Then, import them into Blender, make sure they are all lined up. Then, you can start creating the UV map, which is like the layout for how the liveries will be applied. If you don’t know how to do this, Blender Guru has a pretty good tutorial (just google “Blender Guru UV mapping”). Basically the idea is you highlight all the faces that you want to be able to apply a livery too, then lay them out in a way that you can easily apply textures. Once your UV map is done, export the map (usually it’s just as a .png) and re-export the meshes of the plane to Roblox (this is a must, because the UV maps you created have actual redefined how images are applied to the plane). Once everything is back set up in Roblox, you can use any old art software (like Photoshop, Pixlr, or the like) to create your livery. Once the livery is done, upload it to the meshes’ TextureID, and it should apply!
The method suggested by itsokayepic may require a massive amount of work. Since SkyTech planes are mostly CSG-based. You can try hiring a CSG livery designer through their hub, or purchase a mesh plane and get the designs done on external apps like Illustrator or on paint.net (<= not a link, its the name of the app).
If you want to make the CSG livery by yourself then I would highly recommend you to watch this tutorial to get the concept of how its made.
Mesh liveries may result in a pixelated livery, however they are extremely useful when it comes to complex designs and they can be made much quicker than CSG ones. Here’s are some liveries that I did in case you would like to see how mesh liveries would look like: