I want to sell my hat(s) for metal, how do I turn my hat(s) into metal?

This is the Hat banking page. Click on the "Sell Hats button to begin.

Here you choose the hat(s) you want to sell. I've selected a Madame Dixie.
To the left you can filter your hats if you for example only want to see Demoman or Medic hats. 
To the right you'll see how much you'll get for your hat(s).
Hit the green "Trade Now!" button to continue

You're now in the queue to sell your hat. You can see the queue at the top of the screen. I'm currently in a "global queue" which is what you'll be in at the start. Then you'll be assigned to a bot to trade with it. You might be in a queue for a minute or so. 
You can see what the trade needs to look like. 1 Refined and 2 Scraps metal for my Madame Dixie.

Now this will pop up. Click the green "Go to trade offer"

You'll now get into a trade. Simply accept the trade if everything is alright.
When the offer is alright and has gone through then the trade will be closed.

You'll now see this. You now have your metal. Simply close it by clicking the white X at the top right of the box.

You've now succesfully completed a Hat banking trade! Congratulations!
It's basically the same with all the other trades, but. You can always go and check those tutorials as well.