The description and attributes of the Vishala Yakshini are given in this post- Here. The Mantra Prayog has to be commenced on the morning of the Saptami [seventh day of either the bright or dark half of the fortnight] of the Hindu Month of Bhadrapada.
The Sadhak has to sit under a Gunja Tree [Abrus precatorius] and chant the Vishala Yakshini Mantra given here 100,000 times. After the completion of the Mantra Sadhana, the Sadhak has perform a Panchopachara Puja and perform Dashansh Havan with offerings of Aniseed Flowers[ Saunf Flowers] dipped in Pure Cow Ghee[ Pure Clarified Butter of a Cow]. Each time the Sadhak offers the Aniseed Flower to the Havan, he has to chant the Vishala Yakshini Mantra.
For Panchopachara Puja see this post- Here. Dashansh means one tenth of the number of Mantra Chants, in this case 10,000 offerings. A Sadhak has to use a Mala of Red Coral for the purpose of counting the number of Mantra Chants.
आने विशाले क्रीं ब्रीं क्लीं क्रीं स्वाहा ||
Aaane Vishaale Kreem Breem Kleem Kreem Swaha ||