Make a mixture with pure essential oils:

3 drops Eucalyptus
3 drops Pennyroyal (substitute Citronella oil for cats)
3 drops Rosemary

Mix the oils together and dilute in 2 Tablespoons with a fixed
vegetable oil, such as almond or olive oil. Rub a small amount into your hands and apply to the fur. Store in a dry cool place and apply daily until the fleas are gone.

*Caution when using essential oils on cats – use sparingly since they
like to lick their fur.*