Question about editing single.php

I'm trying to get an email opt-in box at the bottom of each post right after the last sentence of the article basically.

If I embed the Mailchimp form embed code into sinlge.php, where exactly should I paste it?

And, is this the best way to go about doing this?

I'm also considering using this plugin, but I'd rather do it manually if possible

http://codecanyon.net/item/simple-mail-chimp-signup-forms/2395156

Thanks for any advice

www.agentchris.com