Upload the livery as a PNG at 4096x4096 pixels (8192x4096 for 2:1 aspect ratio templates) to #livery-submissions on the Project:Homecoming Discord server, with a message of the format
<car name>
<livery name>
<is this an update to a livery that is already on the server Yes/No>
using Shift+Enter to make new lines in the message
for example