I want to buy strange weapons for metal, how do I buy Stranges?

This is the strange weapons page. Scroll down a little and you'll see all the bots.

This is a bot. Click on it to see all the weapons it has.

Here you choose the weapons you want to buy. I've selected a Strange Pretty Boy's Pocket Pistol and a Strange Cleaner's Carbine.
To the left you can filter your weapons if you for example only want to see Engineer or Medic weapons. 
To the right you'll see how much you'll have to pay for the weapons.
Hit the green "Next Stage" button to continue

Now you'll see your backpack. Here you can choose if you want to trade your Strange weapons for the Strange weapons you selected before or if you want to pay with keys. I want to pay with metal so I don't select anything and hit the "Trade Now!" button.

You're now in the queue to buy the weapons you selected. You might be in a queue for a minute or so. 
You can see what the trade needs to look like. 2 Strange weapons for my 1 Refined and 2 Scrap metal.

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 the weapons you wanted. Simply close it by clicking the white X at the top right of the box.

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