1. Check out the trunk if you don't already have it. CD into the trunk.
svn merge https://subversion.branch.repository.url -r firstBranchRevision:lastBranchRevision .
When the branch is created, the firstRevision will be the last revision from the trunk (i.e., it's not going to be revision 1). The command figures out what changed in the branch and then merges those changes with your current working directory (.).
If there are any conflicts, you could resolve them with the command line tool, or, if you're like me, you can use TortoiseSVN to visually edit the conflicts.