Simply add some CSS to your themes style sheet and then wrap the guest author content within that Div class. It’ll automatically be styled accordingly. A good, quick, copy-paste tutorial.
w•pazo | wuh PAW zo | verb 1 to hit something or someone—esp. over the head—with the power of WordPress. noun 2 the best WordPress stuff. 3 a blog that hits you over the head with the good links to all the best WordPress stuff.