First of all – you need LESS compiler – for Windows, I use winless – please download & install it.

Next, grab the Bootstrap ( I use version 3.3.4, newer version broken when compiling the LESS ).

Extract the Bootstrap to a folder and then create a LESS file called bootstrap.less and paste in the following code:

.bs-wrapper {
   @import (less) url('bootstrap.css');
}

Now open up winless, and add folder where your Bootstrap CSS directory, and then on file bootstrap.less output file, you may name it as wp-admin-bootstrap.css.

Next, you just need to use wp_enqueue_style() to include the generated CSS – wp-admin-bootstrap.css