I want to sell Stranges for metal, how do I sell my Strange items?

This is the weapons page. Click on the "Sell Stranges" button to begin.

Here you choose the Stranges you want to sell. I've selected a Pretty Boy's Pocket Pistol.
To the left you can filter your weapons if you for example only want to see Scout or Demoman weapons. 
To the right you'll see how much you'll get for your Stranges.
Hit the green "Trade Now!" button to continue

You're now in the queue to sell your Stranges. 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 Reclaimed metal for my Pretty Boy's Pocket Pistol.

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 sold your Stranges! Congratulations!
It's basically the same with all the other trades, but. You can always go and check those tutorials as well.