This is now documented in the Debian web site.
So, you heard of this cool BTS feature and you what to use it, but don't know how: here you'll find a quick and dirty introduction.
Ok, first some links
now start tagging your bugs:
user <pkg>@packages.debian.org usertags <#bug> <tagname> usertags ... ... thanks
or user bts:
bts user '<pkg>@packages.debian.org' , usertags <#bug> <tagname> , usertags ... , ...
(always use <pkg>@packages.debian.org user).
You're half-way now; it you want a nice packages bugs list like devscripts (with tag to group up bugs), then you need to use usercategory:
user <pkg>@packages.debian.org usercategory <cat-name> * <place a name here> [tag=] + <tag1> [<tag1>] + <tag2> [<tag2>] ... + <tagN> [<tagN>] + Uncategorized  usercategory normal * status * <cat-name> * severity usercategory <pkg>-sort * status * <cat-name> * severity usercategory old * status * severity * classification
pay attention at the order you write the tags in the usercategory, since it will be the same order the tags will show up on the BTS page.
when you add a tag to bugs, you need to modify the usercategory to treat that tag too (I don't like this, but that's the way it is).
you can add a usertag to usercategory even if no bug is usertagged (yet) with it: it will appears as soon a some bugs will have that usertag.
usertag name can have only alphanumerics, at, dot, plus and dash, while <cat> in <cat> [<tag>] can have underscore too (maybe other chars, untested).