Merge Replication SQL
There's some very good tutorials on the web about this that go in depth and have lots of screenshots.  

I'm assuming you also have to cross untrusted domains, correct?  Usually if the domains are trusted it means some sort of network link with AD/LDAP has been established.  If that's the case, you're looking at:
Merge replication across untrusted domains using web synch

Pull Merge Replication Security with Sql Server 2005 with no Domain Set Up (Largely applies to 2008)
http://www.jstawski.com/archive/2007/03/06/pull-merge-replication-security-with-sql-server-2005.aspx

How to: Configure a Subscription to Use Web Synchronization (SQL Server Management Studio)
http://msdn.microsoft.com/en-us/library/ms345214.aspx

You will also run into questions such as 'do I use a sql user, do I use security certs?  SSL or SQL certs, etc.?'  There's going to be a lot of decisions you'll have to make so be prepared to re-do it several times.