I've never top cropped before, only washed yeast after ferment is done. But from reading and this forum, its pretty basic. After roughly 36hrs you should see a trub filled krausen with hop particles and such. This should be skimmed off and discarded. After another 24hrs or so a thick creamy dense krausen should reform and this is what you want to skim and save.
i'm using buckets so its obviously easy. I've been using my metal spoon (sanitized of course) and skimming the junk off and discarding. When i get a nice thick yeast krausen to reappear, i'll use a little boiled and cooled water in mason jars to collect this part. Skim with spoon, pour into the mason jar with sterile water and seal. Sounds like it yields pretty decent results. I'll let ya know how the 2nd "skimming" goes after i'm there.