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Regarding the preparation of Sarcophaga genitalia:
HERE.
I have made genitalia mounts of thousands of moths and wondered whether that technique would work for flies.
With moths, it is best to work with dry material; fresh genitalia do not make good specimens. So, I took a dry Pollenia male:
1] I cut off the last few abdominal segments (a good idea to have a decent photo of the intact fly before damaging the abdomen);
2] I soaked this piece of abdomen (specimen) in 5% KOH overnight at room temperature (actually 15 hours at 20C). The time required will vary for different species, less time for delicate species, perhaps longer for large tachinids.
3] I transferred the "specimen" to 4 changes of clean water, about 5 mins for each;
4] I transferred the "specimen" to a mixture of alcohol & glycerine (66 parts of 50% alcohol, 33 parts glycerine);
5] I let the alcohol evaporate until the "specimen " was just in glycerine;
6] I dissected out the genitalia, all this meant was to break up and discard the abdominal tergites;
7] tranfer to fresh glycerine and take a photo.

The actual genitalia shows amazing clear details, far better than this photo.

This techique may prove useful when one has more than 1 specimen of the same species so that one can 'sacrifice' one fly for a genitalia preparation.

As to the parts: 1 = epandrium; 2 = cercus; 3 = surstylus; (I think:()

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I do almost always use a way simpler, faster method:

1. Remove the abdomen or cut off the last segments - check the relevant literature beforehand not to damage important structures! Sometimes it is essential to remove the whole abdomen.

2. Soak the abdomen in warm water with detergent for some time (a few minutes is enough for small flies, 1/2 hour or more for large flies)

3. Dissect and examine in water (or glycerine, but then you have to wash before the next step or permanently store the genitalia in glycerine).

4. Drain the examined genitalia and glue them on a slip of cardboard, which can be pinned under then original specimen. Once the surface pollinosity of the abdomen is properly visible, the abdomen is dry enough for mounting.

Dissecting is slightly more difficult as muscles have not been obliterated by KOH, but on the whole, this system is way faster than doing a 'proper' genitalia preparation and yields perfectly usable recults. And as a bonus, the cardboard-mounted genitalia and very easy to examine and don't get separated from the specimen.

Using KOH is overkill for mere identification purposes.