This is a guide that my friend in real life made and asked me to post here for you folks since he doesn't have an account here. His name is <Sawbuck> in game.
Your Basic Guide to Playing the Engineer
by, ..
Intro
Hello and welcome to Your Basic Guide to Playing the Engineer, by me, Jake ".." Wedlick. I decided to write this guide for a couple of reasons. Mostly because I will go into a server with four Engineers, and all of them doing a crap job. The Engineer has the best defense in the entire game, and is crucial for holding control points and securing intel. This will teach you how to make the most of the Enginner.
Weapons
Shotgun: This is your primary weapon. At close range it rivals the Scout's scattergun or even the mighty flamethrower. It's about as basic as a shotgun gets: if they're close then shoot, if they're far away forget about it.
Pistol: This is your backup. If they're too far away to hit with the shotgun then you can try this. It's fairly powerful but being single shot it's hard to hit anything smaller than a demoman, so think of this as a last resort. You're better off running or getting closer and pulling out the shotgun.
Wrench: This is the weapon you will have out most of the time. It's used to upgrade your sentries, repair structures, remove sappers, and build structures faster. Coincidentily, it's also my favorite mêlée weapon. A whack with this will make a spy think twice before messing with you.
Build menu: Self explanatory. This is where you build stuff. Sentries are 130 metal, dispensers are 100, and teleporter entrances and exits are each 125.
Destruction: Also self explanatory. This blows up your structures. You gain some metal back from this. Use if you have misplaced something, want it moved, or to be a jerk and deny points to people.
Structures
Before I get into the different structures, I would like to start off with this: Never, ever, build out in the open. A Soldier or Demoman would be happy to turn it into scrap metal. Always build behind a wall, rock, or anything that blocks direct line of sight. This is the number one mistake nooby Engineers make, and they usually end up back at the spawn before the structure is done building. With that said let's move on.
Sentry: This is your primary building. Guard it with your life because when your sentry dies, you are virtually useless. Always build these in or around corners. Also try to put it somewhere unexpected so when the enemy is blitzing down the hallway they get some rockets to the face.
As an Engineer, you want to pick a building, hide the sentry so it's not in direct sight, then sit and wait for the enemy to come in. The room should only have one entrance from the enemy's side and shouldn't be easy to shoot rockets or grenades into. This room is going to be known as your Sentry Room, get comfortable here because this is where you're going to camp for most of the match.
They cost 130 metal, then 200 metal to give it bigger guns, and another 200 metal to give it rockets. Always stay by them because they are a primary target for the enemy. The cost of metal to repair and restock them varies on how much health or ammo has been used.
Dispenser: There are a few good places to put these. Most Engineers put them right next to them so they have a good supply of ammo for their sentries. If you build your sentry near the spawn and can restock your ammo supply quickly, then build them on the front line or next to the snipers. But always remember: never build out in the open. They are well worth the 100 metal because as it charges it releases bursts of 40 metal and a constant stream of health.
Teleporter entrance: These are obviously to teleport your teammates (or enemy Spies) to the front lines. Most Engineers build this first, directly outside the spawn door. This is usually ok, but once in a while a Spy will sneak around and have a f