Proper filling of pierogi starts with the filling itself.
The filling should be stiff and chilled. This allows you to put enough filling into the pierogi and to control it while you seal the pierogi.
You do not want any moisture in the filling that would leak out during the sealing process. Such leaking would affect the seal on your pierogi and they might open during the boiling process.
If you are using a fruit filling which tends to be watery, add some cornstarch or potato starch or breadcrumbs to the mix to thicken it.
To fill the pierogi when making by hand, put the circle of dough in your hand and then add the filling.
Do not place filling on all of those circles. Fill them one at a time while you hold the circle in your left hand and then use the pinky on your right-hand to press the filling into place.
Once the filling is properly in place, then fold the circle and make the first crimped pass. Thereafter you can braid the pierogi, crimp it or fork seal it. See how to seal pierogi.
If you are using a pierogi press, once again put the filling on each dough circle as you put it on the pierogi press. See using a pierogi press.
And, of course, if you are using a pierogi maker, you fill all the pockets at once prior to adding the second layer of dough. See using a pierogi maker.
When filling pierogi, make them as plump as you can. Remember that people eat pierogi for the filling and not the dough. So give them ample filling in a pierogi that does not have a large sealing area.
After you have made your pierogi, leave them on the pastry board and flatten them. By flattening them you ensure more even cooking. Plus the pierogi look nicer.
The most popular fillings are the potato and cheese filling, mushroom filling, spinach filling, blueberry filling, and cherry filling.