Rick said:
Crossposting is done by listing more than one group in the Newsgroup
entry of your message when you post it. Just separate them with
commas. This is much preferable to Multi-Posting where you post
duplicate messages one after the other to different groups.
*Excessive* crossposting is also discouraged however which is listing
a high number of groups. Generally three or four groups is the
maximum you should use.
Just to add to that. Posting the same message to several groups, when
it is appropriate to do so, and doing it one group at a time denies those
participating to see the replies posted only to the other groups. This
usually will mean less useful information all around.
Excessive cross posting means you are likely choosing inappropriate
newsgroups. For the most part one or two will fit the subject well and the
others are really off topic.
In either case and often with the same message, you may get complaints
about what you do. Don't worry. Just try to learn and choose the method
that seems best to you.