This is best done on a clean countertop or table. Start out by portioning out your flour in the middle of your work surface and mix in salt.
Make a well in the middle of the flour and add in your egg and sour cream.
Slowly add in your water and mix in the rest of the ingredients, drawing the flour from inside the well.
Continue doing this until all the ingredients are mixed, kneading until a dough forms. If the dough is sticky, add a bit more flour. You don't want your dough to be so dry that it crumbles or so wet that it sticks to your hands.
Divide up the dough into 4 pieces. Work with one piece of dough at a time (cover the reserved dough and set aside). Roll out dough until it is about 1/8 inch thick. Dough should be slightly thinner than pie crust.
I use a pierogi mold (makes 14) instead of making individual pierogi by hand. Sprinkle flour on the mold so the pierogi doesn't stick. Lay one side of your rolled out dough over the mold, making sure you allow enough dough to fold on top of the pierogi once you fill them.
Fill each pierogi with your desired filling, forming pockets by pressing filling down slightly.
Be careful not to overstuff the pierogi, once filled, fold the other half of dough over top of the mold and using the rolling pin to seal and separate the pierogi.
Remove excess dough around the edge of the mold, flip the mold over, and pop the pierogi out of the pockets.
Repeat process 1-2 times with excess dough