Skwarki are bits of fried bacon and onion. Your pierogi will come out best if you fry them IN the bacon grease with the onion, so they can absorb some of the amazing flavors.
It's quite simple - chop up some bacon and some onion, drop 'em in a pan and fry them over medium heat. Drop in the (at least partly) thawed pierogi*, adding extra bacon grease if needed. Fry for 2-3 minutes, or until heated through and, you know, fried looking. Transfer to a plate or a bowl and enjoy - the skwarki are an amazing compliment to beef pierogi in particular, but are also a great compliment to the more mundane varieties, like potato.
Burning the ever-lovin crap out of the skwarki is optional. But tasty.
*I really like the ones from Kasia's Deli, which can be found in some Chicago supermarkets (or purchased from their store on Chicago and Hoyne). Their webpage (which is kind of charming in a Mom and Pop, nothing-but-the-basics kind of way. They've posted letters from their fans. Actual pictures of the letters. And not just from famous people, either. It's great.) seems to imply that one can perhaps order them from out-of-state as well. If you can, go for it, because Kasia makes damn tasty pierogi.