1) Create your Key pair, as per normal. Typically, this can be done as:
Sn.exe -k PublicPrivateKeyPair.snk
This will create a file containing both keys.

2) Extract the public key:
Sn.exe -p PublicPrivateKeyPair.snk PublicOnly.snk

3) Calculate the token. Note that the ‘-t’ parameter MUST be lower case (‘-T’ does something different)
Sn.exe -t 'PublicOnly.snk'
And there you have the public key token