Download bootstrap css and js files and put them in webroot/css or webroot/js respectively.
Then in app/View/Elements/header.ctp file add
<link rel="stylesheet" href="<?php echo Router::url('/css/bootstrap.min.css', true); ?>">
<link href="https://stackpath.bootstrapcdn.com/bootstrap/3.3.7/css/bootstrap.min.css" rel="stylesheet" integrity="sha384-BVYiiSIFeK1dGmJRAkycuHAHRg32OmUcww7on3RYdg4Va+PmSTsz/K68vbdEjh4u" crossorigin="anonymous">
And in app/View/Elements/footer.ctp add
<script type="text/javascript" src="<?php echo Router::url('/js/bootstrap.min.js', true); ?>"></script>
Now you can access bootstrap classes in your code.