You can just do it like a regular link, of course - use the Insert Link button if you use the Rich Text editor, or <a href="http://whatever.dreamwidth.org">whatever community</a> sort of thing if you use the HTML editor.

If you mean doing it up all fancy with the DW-style userhead/community icon, since LJ lacks DW's nifty ability to do <user name="whatever" site="livejournal.com"> and have it automatically done for you what I usually do is go to the journal on DW, highlight an example of the userhead and journal name anywhere you find it on the page, and then right-click and View Selection Source.

Strike all that. It used to work differently. Sorry for the multiple edits. I did test this comment before posting it the first time, but I should have been testing it on LJ, not DW. The below is still fine though:

Because I have crossposting set up from DW, I've also gone the route of posting a "scratch" private post containing just the journal I want to link to, then editing the crosspost on LJ and copying the code from there (and then deleting that post), which yields slightly different code:

<span style="white-space: nowrap;"><a href="http://ao3some.dreamwidth.org/profile"><img src="http://www.dreamwidth.org/img/silk/identity/community.png" alt="[community profile] " width="16" height="16" style="vertical-align: text-bottom; border: 0; padding-right: 1px;" /></a><a href="http://ao3some.dreamwidth.org/"><b>ao3some</b></a></span>

But the effect is the same: ao3some