Domain mapping subdirectory (MU question)

I have a WordPress MU install at http://my.blog.host.tld, I want the different blogs appear under http://www.domain.tld/blogs/ub/+blog…

http://www.domain.com/blogs/ub/ forwards requests to the http://my.blog.host.tld address fine, but I wonder what I have to do in order to get the WordPress MU install to work properly?

Do I a) have to do something during the WPMU install? or b) post install? If so, what?

I have found a few questions about this, but no answers that correspond with my understanding of the problem. A point here is that I may just have too little understanding of domain-jiggerypokery and/or WPMU. Any help gratefully received!