Where you list the newsgroups in your newsreader, you add them manually,
separating them with a comma (but no spaces). Like so:
rec.autos.makers.honda,alt.autos.honda,alt.autos.acura
Do not post to any more than three groups. All the groups should be related
to your message in some obvious way.

Failure to cross-post means readers in one group who are unaware of the
same post in another group will miss any replies and information posted
only to the other group. Cross-posting ensures replies are seen by all, so
all may be enlightened (or enraged or amused as the case may be...).