Hi Fara,
You would use the heating method outlined in the guidebook to infuse your cocoa butter with yarrow. The instructions are for a Comfrey Infused Oil which is found in the Herbal Body Products Course section as well. The only difference being that, instead of letting it cool fully, you will strain the cocoa butter while it is still warm and liquidy.
I hope you have a great time making it!
Warmly,
Benna