MAT12 matched pair transistor substrate connection
From the datasheet:




  
  Substrate is connected to case on TO-78 package.
  


Now, here is a TO-78:



As you can see, the case is actually metal.

If you necessarily want to connect the substrate, the recommended way is to do it using a heat sink. I'm not sure if you actually gain anything by doing that. It could be feasible to solder a small wire or pin to the tab, but I've never done it, and I would be afraid that it could damage the matching. As Tom points out in the comments:


  basically if you use a metal heat sink with no insulation, don't connect that heat sink to anything but the most negative circuit potential


I suspect that the only reason they mention that you can connect the substrate is because you sometimes have to, due to the heat sink. Not because they think it is warranted. If it gave any performance benefits, I'm sure they would point it out.