Step 1
Sign up for hosting somewhere. You need to have PHP and MySQL installed, at least.
Dreamhost will work nicely, and quite often, you can signup, with a unique domain name for just 10 dollars, for the first year.
There are many other hosts to signup with. It will often run about 5-15+ dollars a month. If you are not getting mega ammounts of traffic, a host somewhere within this range should suffice.
Step 2
Install MediaWiki. This can have a bit of a learning curve, but once you get the hang of it, it is not super super difficult. See:
http://www.mediawiki.org/wiki/Manual:Installing_MediaWikiStep 3
Get the XML dump of the wiki that you wish to fork.
Many can be found at:
http://download.wikimedia.org/backup-index.htmlIf you want to fork Commons, then see:
http://yousefourabi.com/blog/semantic-web/...ages-with-wikix If the wiki you wish to fork does not have an XML dump available, then you still could fork the wiki, but it could be a pain. You would have to get the page list at [[Special:Allpages]] and then either export all pages manually at [[Special:Export]] (with a large wiki, this could take a long, long time) or else setup some sort script to do it (find it using Google, if it exists, or write it yourself).
Step 4
Import the dump. See:
http://www.mediawiki.org/wiki/Manual:Importing_XML_dumpsAppendix
Configure the Wiki
Then of course, configure MediaWiki at sometime in here. You may want to Import [[MediaWiki:common.css]] from the big wiki.
You can enable the use of images from Commons. See:
http://www.mediawiki.org/wiki/Manual:$...oreignFileReposA pretty decent overview of other good to know configurations are at:
http://wiki.dreamhost.com/MediaWikiYou may need to tweak the configuration over time. See:
http://www.mediawiki.org/wiki/MediaWikiThe MediaWiki wiki has a lot of information on it, and is organized perhaps pretty reasonably.
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