It's best to wait a few weeks after shearing for there to be a little bit of fleece for the fly prep to stick to. Flies like fleece, and the girls will be bare, so they aren't at risk for a while

The flystrike preventative we use is Crovect. Here we do not routinely treat adult sheep, only lambs, as the adults have developed some protection, plus they get shorn which massively reduces the risk. It is very important to read the bottle carefully, to use the correct nozzle and apply it as appropriate for the treatment you are giving. (It can also be used, but differently applied, for treating actual strike, and for ticks and some lice.)