OK, you did say you wanted step by step. Thing is I haven't tried it so I could step by step you into something that doesn't actually work. But here goes:
1) Edit postbit_legacy, and where you want the similar threads to appear (at the end I suppose), insert this:
Code:
<vb:if condition="$post['isfirstshown']"><!-- similar threads --></vb:if>
2) In the adminCP go to Products & Plugins > Add New Plugin, and find "showthread_complete" in the Hook Location dropdown.
3) Enter a title, something so that later you'll remember what this plugin is for.
4) In the large "Plugin PHP Code" box, enter this:
Code:
if (isset($similarthreads))
{
$postbits = str_replace('<!-- similar threads -->', $similarthreads, $postbits);
unset($similarthreads);
}
5) Select the Yes radio button next to Plugin is Active at the bottom, and click Save.
There's a chance it won't work. If it doesn't, post here and maybe I'll have to actually try it.