Every potion comes with a unique puzzle shape that needs to be completed in your cauldron. Combine and arrange puzzle pieces using the ingredients you have in your inventory. 

Some potion recipes impose restrictions on the type of ingredients you can incorporate. Follow the instructions meticulously to initiate the brewing process.

Keep an eye on the circles next to the cauldron; they indicate the maximum number of ingredients you can use. Exceeding this limit will prevent you from commencing the brewing process.

You can learn more about it here : https://potion-permit.fandom.com/wiki/Brewing