This is for the regular TF2 player. If you don't know what TF2 s it is a free to play fps with nine classes (spy, sniper, engineer, heavy, medic, demo-man, scout, pyro, and solider). It has many different game modes such as capture the flag, control point/king of the hill and payload race. Sorry if I misspell anything and let's get started on this tutorial.
Step 1: Getting Started...
Get in a tf2 game mode most helpful in capture the flag or any of your preference.
Create a load out with a PDA for the sentry gun and a good wrench.
From there you need to join a game and find a safe place to set up.
Set up your dispenser first so that you can get metal to build the sentry or upgrade your dispenser.
Once you max out the level of the sentry pick it up and prepare for the next step.
...Continue on next step...
Step 2: Prepare to Run
Start the run with the sentry in your hands.
Make sure to place the sentry along the way but place it before you see a enemy because it does take a while for the sentry to set up and be prepared to protect itself and you. If you place to late your are pretty much asking for a death.
(If you do die then you have to start the sentry over unless it wasn't destroyed.)
Once you make it place the sentry down and wait once you capture anything relating to the game mode you are in.
Grab It and begin the run home
Step 3: Repeat
Keep going with steps over and over again until you win or decide you tired of being engineer. Not even I can play as engineer until my team wins. But most of all look out for the enemies you don't want to be surprised when a defensive engineer sees you walking around the corner. So good luck with your trials and your success. Most of all have fun playing tf2.