Just go to https://store.steampowered.com/steamaccount/addfunds then select the amount you would like to pay.
On the payment screen change your payment type to bitcoin. If it has selected something else already you might have a tiny 'change' button next to your last payment type, click that to select bitcoin instead.
Once you select bitcoin, update the form information and click 'continue'.
Now you'll be redirected. If nothing happens, try clicking the link. If that still doesn't work you can try refreshing the page as well. Once the redirect is complete you'll be shown your BitPay invoice. We recommend taking a note of the invoice url, order ID, and the bitcoin address of the invoice just in case you need to locate your order later.
Simply pay and your credit will be applied to your Steam account as soon as it's confirmed. If you need any help paying a BitPay invoice, have a look at How to Pay with Bitcoin.