Brzuszekpronounced Bzoo-shehk with the z sounding as if you are trying to say "Sh" or "r" at the same time*.
Pępuszekpronounced Pehm-poo-shehk with the sz sounding as though you are slurring (imagine how inebriated people say their S or Sh, if that helps).
Make a circle with your finger or palm around baby's belly while saying slowly "Brzusiek".
Then, gently touch the belly button while saying "Pępuszek" quickly.
Baby will laugh at you, this is a great thing. When babies laugh, they are enjoying learning from you.
When my second daughter was born, my older daughter played "Brzusiek Pępuszek" with her little sister. Sometimes, they still play that game together, along with "Idzie Rak" and "Round the Garden with Teddy Bear".
I remember being a little girl and my mother playing "Brzuszek Pępuszek" with me and later adding it to a Polish version of "Head Shoulders Knees and Toes".
* The "rz" and "sz" sounds are very difficult to describe and sounds which need much practice.