While playing, bring up the "Message:" prompt by pressing enter, type "ophelia" and press Return (don't include the quotes). Bring the prompt up again and type "x<race name> <mission number>" to jump to the specified mission (again, no quotes).
For this glitch, all you will need is any flyable terran building, and a seige tank (enemy, or your own). First, you will need to make your building liftoff. Normally, when a seige tank is deployed, and you try to land on it, it will show red under the placement and say that you can't land there. You will need to land it on a deployed seige tank. To do this, you need to predict where the seige tank will actually deploy. Before, it gets there and deploys, you need to tell the building to land. If it is done right, the seige tank will go where it was told, and deploy while your building is landing. The building will land right on top of the seige tank. If it was any other unit, it would just be pushed out of the way by the building, but since a seige tank cannot move while deployed, the building will land, and the seige tank will be destroyed at no cost to you.
First make sure you have at least one dark archon with the mind control upgrade. Then, max out your population. Now any units you mind control will go over your population limit but will not affect you.