I have created a guide for all of Don't Forget's Golden Grind Room doors, complete with screenshots of their locations.

Ruins: The ruins grind room is in the room after where you fight Nabstablook in regular Undertale.

Snowdin: The grind room in snowdin is in the room right after the enterance.

Waterfall: This time the grind room is right at the end, when you reach the warp point in waterfall, continue on right, and you should find the hallway that holds the door.

Hotland: As Hotland is pretty easy to get through, you should encounter the room that has the door anyway, you can refer to the image for more info.

New Home: There is no grind room in new home, as it does not have any encounters.

Dark World: When exploring the dark world you will come across a room, picture above, that will have an odd shape in the edges of the platform. Walk into this area to reveal a secret path that holds the final grind room door.

(Note that the grind rooms cost gold to open)

Thank you for reading this guide and I hope it helped you.