Are you asking how to set nameboards in-game or adding them to existing models?
If it's the former, nameboards only appear on models created with them. Out of all the LNER and GWR models, I believe only skipper1945's King class locos can equip and unequip a nameboard by hitting the pantograph button.
johnwhelan has created some gorgeous GWR passenger stock that also have nameboards matching that of the Kings.
Outside of the GWR and LNER, lielestosbrat made a rebuilt Merchant Navy class model for 06 that includes a script to equip several different nameboards for various Southern named trains.