Domain Forest trust and Vpn

hello they,,I have a problem.We have two offices some distance apart.There is no connection say fibre optics or something like that.I Created a domain(say cyber.local) in one office and another domain (say partnter.local)in the other office.Each office has its internal DNS.We the same ISP.
I was thinking of creating a VPN But am not clear with its creation and am very ready for guidance,,someone told me that i need to create a trust between the two domain forest inorder for them to communicate.I need guidance to all this,I will appreciate a step by step guide.or any other idea that can work.