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First, the account must be created. That process is detailed in the PMC FAQ http://www.apache.org/dev/pmc.html.
Root Infra no longer grants Karma unless specifically requested (usually because the PMC Chair is inactive or unresponsive), as outlined in the PMC FAQ, but it's easier for the PMC Chair to do it.
Basically, the PMC Chair needs to edit the asf-authorization file which is stored in svn at https://svn.apache.org/repos/asf/infrastructure/trunk/subversion/authorization/asf-authorization. The file format is fairly self explanitory, but documented at http://svnbook.red-bean.com/en/1.1/ch06s04.html#svn-ch-6-sect-4.4.2
Oh, and don't break that file, as you won't be able to commit to fix it. 
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The instructions that used to bne on this page are now obsolete. The current procedure is now documented in a section of the above link, https://www.apache.org/dev/pmc.html#SVNaccess