External CSS style sheet

I am building my own theme. I am trying to link my external header.css style sheet in my html file, but can not figure out why it is not working. header.css is located in wp-content/blank-Theme/css/header.css I have tried the normal way of linking….

<link rel="stylesheet" href="css/header.css" type="text/css"

but that didn’t work. I have done a search on the forums and found one that matched what i was trying to do. The answer recommended doing this bit of code below, but even that didn’t work…

<title>Untitled Document</title>
<link href="<?php bloginfo('template_directory'); ?>/header.css" rel="stylesheet" type="text/css" />
</head>

Does anyone know how to link an external style sheet? My style.css is getting way to overcrowded. My website is timwilsonsblog.com (if you need it) , but it manly just has filler text and still needs a lot of work, so don’t judge me ;D