Its a drag to build, had to look it up.
You have to place it realy close to the cauldron, almost inside of it for it to be ''close enough'.
And it has to be on a roof or beam part, this can be horizontal or diagonal.

Hope this helps once you can build it.

you discovered turnips?
And thistles?
And carrots?
And Mushrooms?

Look for Dandelions, Carrots, Mushrooms, Thistles and Turnips and you'll get the option, enjoy! =)

yes ty