Not much can be said about this mission. Describing your exact path would take a really long time and a lot of space. So, I'll just offer a few tips.
As you proceed through the level, always keep your forces tightly packed together. You will meet a moderate amount of Terran resistance, and you might be overwhelmed if you don't keep your men together.
There are several things to remember. First, it is important that you walk over every single beacon you see. 
Second, always lead with Raynor, following with Firebats tightly, and then Marines right behind them, until Raynor has less than 50 health. Then, put him in the middle of your group. Also, in a big room at the end of the hallway you see behind the door next to you as you start out, there are 6 Marines and a Goliath. Proceed cautiously and keep your forces together in the order mentioned above. Try not to use Stim Pack until you reach the last room of the mission.
