Blender

Blender is a free 3D Modelling software. You can use it to create all the parts of your map, including meshes and UVs. This wiki won’t teach you how to model, but you can find a lot of tutorials online and on Youtube.

PSK Plugin

Exported Skeleton Meshes from Rocket League files are .pskx files. To open them in Blender, first rename the file to .psk, then go to File > User Preferences > Add-ons then search for Import Unreal Skeleton Mesh (.psk)/Animation Set (psa) and enable it.

Rocket League UPK Decryptor

RLUPKTool is a tool to decrypt .upk files. Normally you wouldn’t be able to use .upk files from Rocket League but with this tool you can decrypt and then import the decrypted files _decrypted.upk to other softwares.

Umodel (aka UE Viewer)

UModel is a free software to preview and extract content from Unreal Engine files. You can load Rocket League decrypted files in Umodel by dragging and dropping them on the umodel .exe. Once it is open, you can navigate between the assets with your PageUp and PageDown keys. The shortcut to extract an asset is Ctrl + X. It will be exported to a UmodelExport folder.

Photoshop / GIMP

You’ll need an image editor to work with textures. Unreal Engine textures work best if their dimensions are a power of two. To make sure UDK will let you import them, you can resize and scretch your textures in your image editor to fit them in a 512px, 1024px, 2048 or even 4096 square. The bigger, the heavier.