Do you have RDBM?
I usually open up a Career Save file in RDBM and then open the teamsponsorlinks table in the DB2 tab.
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Here you can assign adboards using existing ones. For Example: if you wanted to give a team the existing FedEx adboard in the game you would create a new artificial key (1660 or whatever number is next), add their teamid (For Example: Arsenal =1), and then if you click in the adsponserid cell there should be a drop down option with existing options - you can select those there.
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If the image is in the drop down you usually don't need to assign a dynamic image id so just put 0. Make sure you put 1 under isapproved or it won't show up in game.
If you want to create a new adboard you can go to FIFA Editor Tool > Legacy Explorer > UI > Game > adboards
I like to export a simple adboard like FedEx because it only uses one .dds file
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Export this .dds file and replace it with the image of your choice and save it as a mod. Then repeat the steps above to get it to show up for a specific team.
Remember that if you replace the FedEx file then the FedEx logo won't show up anymore in the game - it will be whatever logo you replaced it with on a white background. If you don't want to replace FedEx you can export the FedEx .BIG file and import it to any of the other adboards in game - for example these FUT ones.
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Some BIG adboards have two .DDS files - a background color and logo - I like to edit these because you can easily create a simple board with a logo and a simple color.
Hopefully these instructions are somewhat helpful! It's a lot of information at first but after you do it once or twice it's easier!