Basically, it is required 3 steps to create BDC in MOSS 2007:
1. Create an application metadata XML file
2. Import the file to SSP
3. I recomend to use the utility - BDC Meta Man the link: http://www.bdcmetaman.com/knowledge%20base/MOSS%20BDC%20-%20getting%20started.aspx
it is free (currently) for a limited edition.