Assuming the key is the posted compressed WIF KwDiBf89QgGbjEmUcXPRwPuKPFnFECvJkvovGNiyjaujMCrQXy98:

1. Decode the key you have using the encoder used (in this case base-58)
   80-00000000000000000006f5d7d806edef2e1110b0b243f9a90c2aaaaaaaaaaaaa-01-098dbf37
2. Remove the extra bytes (version byte, compressed byte if present, checksum)
   00000000000000000006f5d7d806edef2e1110b0b243f9a90c2aaaaaaaaaaaaa
3. Take the remaining bytes (in this case it must be 32 bytes) and convert it to an integer assuming it is positive and in big-endian (most significant value is stored first).
   666666666666666666666666666666666666666666666666666666