Standard welcome/warning messages are both cordial and correcting. Consider using these templates for welcoming, or the first two here for warning.
Try instead to follow the points set forth in this article to relieve new editors of their ignorance. Keep in mind that this is not the way many other things work, and even seasoned editors fail to follow—or are simply unaware of—our guidelines from time to time.
To a newcomer, the large number of is overwhelming. Ignorance of the rules can often be expected, but willfully disregarding them and disrupting the editorial process of constructing our online encyclopedia is quite another. If you exclude editors without barnstars and the like from your circle you probably diminish the final product.
In all cases though, we ought to interact with our fellow editors with gentleness and respect. This is the most important thing to stress. Some strategies are:
If you have bitten someone, or feel that you have been bitten, considering the following points could help ensure that it doesn't happen again.
One common error among newcomers is to create an article in mainspace about themselves, their garage band, or about their original theories on a certain topic. One way to deal gently with this is to userfy the article, and leave a note saying why. {{}} is designed for userfying autobiographical articles. The remaining redirect can be flagged for deletion using {{}}. Userfied articles on bands could be tagged with {{}}, since they tend to hang around. New articles about a person's original research and theories could have a note appended explaining WP:OR. It is sometimes helpful to direct new users to alternative outlets.
Another common newbie error is to violate the three revert rule. There is no reason to expect that a newcomer would know about this rule, so it is a good idea to inform them of the rule on their talkpage after their second revert.