Hello,
What link Vikas gave is generally valid for the NS-208 as well but there is a possibility that some syntax for NAT may be different on old NS-208.
If some commands do not work on NS208, you can go to link below and extract the 'Address Translation' PDF for 5.4.0 or whichever version your NS208 is running on.
https://www.juniper.net/documentation/software/screenos/screenos5.4.0/
Regards,
Rushi