Hi, maybe you can ask Johannes Jehle: http://www.jki.bund.de/no_cache/en/startseite/institute/biologischer-pflanzenschutz/personal/dr-habil-johannes-jehle.html
It depends on which pest insect to isolate the virus from and which type of application you will try but anyway it simply based on homogenize the infected sample in favorite buffer and use serial centrifugation started from 1000 rpm to 10000 rpm to have the virus(polyhedra) in pellet. If you want to isolate and purify the virions it is different method and protocols.