How do I attach these two doorknobs to a piece of board?

Hi Linda! One of the cheapest ways to do it, is to take your knobs to the hardware store and see if someone can help you find in the specialty screw section the right size hanger bolt (aka double sided screw like this: https://www.lowes.com/search?searchTerm=hanger+bolt) to fit. If the coarse end of the screw is longer than the thickness of the board, the excess can be cut off. The hole will need to be predrilled.

Another option is to buy these which are made for vintage knobs https://www.amazon.com/Renovators-Supply-Doorknob-Spindle-Vintage/dp/B077GDBQJ5/ but sizes vary on different knobs so it can be difficult to shop online for that kind of attachment.

Linda: We'd love to help, but need more information about what you need.

Remove the rod between the knobs by loosening the tiny set screws on knob. Get a bolt to fit insead. Drill hole in board the size of bolt, add knob, tighten set screws.

here ya go--- https://www.youtube.com/watch?v=eIyePin2wKQ one more https://www.youtube.com/watch?v=XvSGNttqVHw one more https://www.youtube.com/watch?v=wsRPM7D6tG4