Root link for all VPN confg and troubleshooting.
This is quite a good place to start.
As well as:
http://www.juniper.net/techpubs/software/screenos/screenos6.2.0/ce_v5.pdfIts the VPN guide for screenos 6.2 which does have some good examples there also.
If you are not running 6.2, then go have a look at the
http://www.juniper.net/techpubs/software/screenos/ link and select your version.