Was it this one: https://www.hometalk.com/37575227/tufted-headboard-slip-cover?expand_all_questions=1?

Tufted Headboard Slip Cover

https://www.hometalk.com/37575227/tufted-headboard-slip-cover

https://www.hometalk.com/41223326/build-a-diy-headboard-and-create-the-bedroom-sanctuary-you-deserve

Tufted Headboard Slip Cover

Build a DIY Headboard and Create the Bedroom Sanctuary You Deserve

Hello. This Headboard upholstery Links might be helpful.

https://www.hometalk.com/41921616/s-tufted-headboard-ideas

additionally

https://www.hometalk.com/3587377/diy-upholstered-bed?

DIY Upholstered Bed

I tufted my headboard, but used plywood. I can't imagine poster board would hold up. I pre-drilled the holes in the plywood, and then used a 6" needle and carpet thread to add the buttons..... https://www.youtube.com/watch?v=lsk_MJZXMC0

here is how I did it - https://www.charlestoncrafted.com/turn-a-wrought-iron-headboard-into-an-upholstered-headboard/

https://www.youtube.com/watch?v=msKtP4hOPMw