VPN Setup SBS 2003 Premium

I need some help being pointed in the right direction.

I have sbs 2003 premium with ISA 2004. I need to allow vpn access to this network from another office. I have read everything I can find and can’t get anything to work. I am using pptp and everything I do in isa port 1723 says closed when I do a port scan from outside and when I try to connect to the vpn I get the #800 error. Any help would be greatly appreciated!!! Is there a step by step procedure that I can get somewhere. Thanks.