Each stage splits the datapath into two halves. The right half goes through the F function together with the key.
The output of the F function is then XORed with the left half of the datapath.
The function F replicates some of the bits from the datapath and ignores some of the bits from the key to obtain two 48-bit words that are XORed.
The XORed result goes through another block called the S-box. An S-box is a group of 8 lookup tables with 6 inputs and 4 outputs each.