Signs and Portents: customising signs, billboards and flags
Everything about customising signs changed since version 1.13.
Read This! – it is a great tutorial, I could not possibly write it half as well. It tells you everything you need to know about creating your own road signs.
SIGNS BILLBOARDS and FLAGS
how to avoid a crash on saving sign template
making custom signs
make your own sign template
how do sign images work
tobj editor
make custom billboard