First wear a mask!
Silkie said DE worked for feather lice so you could try that for them.
For dusting, the best method I have seen and people like was the shake and bake method. Put dust into a small garbage bag, put the chicken in up to the neck and shake. Keep it sealed enough that the dust does not get into the eyes and mouth of the chicken.
The liquid spray is one under each wing and one near but not on the vent. I would spray where you see the lice too.
The whole flock needs to be treated and the coops need to be cleaned and treated. If one has it the the others are assumed to have bugs too.
Ok so I have LF and Seramas, also chicks under a month old and also chicks that are feathered out. I'm assuming all of them have the lice. I looked under the chick's wings and they definitely have the lice crawling around...they looked like fleas to me. Gross!!!! What would you recommend with chicks? How much of the spray would you use on them? Regarding treating the coop, does that mean spraying all the wood surfaces with the Seven spray and replacing bedding?