I want to buy a hat for metal, how do I buy hats?

This is the hat banking page. You can view a list of all available items at "Buy Hats"

Here you can click on the cosmetics which you want to buy and they'll appear at the top. For this example, a Pomade Prince and a Tiny Timber were selected.
The filter button at the top-left allows you to filter the available cosmetics by class and/or slot, and the total price is displayed on the right.
If you would like to pay using just metal and keys, you can now hit the shopping cart icon to be added to the queue. If you would like to pay with your own cosmetics, you can instead press the inventory button and select the items you would like to pay with, after which you can press the shopping cart icon to get added to the queue.

Once you're in the queue you'll have to wait for the bot to send you the offer.

Once the bot has sent the offer, this popup will appear. To continue the trade, click on "Open Trade Offer" and the steam tradeoffer will appear.

Please check if the offer is correct, and then confirm and accept the trade. You will now have to confirm this trade within the steam authenticator.

Once you have confirmed the trade, the site should update shortly after. The items will now be available in your inventory. Congratulations on a successful hat banking trade!