[squishi]

The way to do this is to look at what php page you are loading.
So it depends where you want to insert the template.

Let's say you want to insert a template on a thread's page somewhere in the SHOWTHREAD template.
So you would be looking at the thread with the showthread.php file.
This is the file that you have to scan for hooks.
Open it in a text editor and check what hooks are in the php file and where they are located.
Often, there is a hook at the end which is safe to hook into (in this case 'showthread_complete').

Then you do as suggested above. Go to the plugins. Create a plugin for the hook you just found. Eval the template in that hook and use the variable in the showthread template.