In order to do a manual, you're gonna wanna feel comfortable leaning back on your tail. It's easy to find a balance, but what will help you a lot is either holding onto a buddy or a fence in order to find the perfect center. If you can, have you friend hold onto the nose of your board so you can find a bit more of your center without feeling worried that you might fall backwards. Now, what generally helps with this trick is feeling comfortable knowing that your back leg is stiff and your front leg is gonna be slightly bent. So, some people like both legs stiff, some people like one knee bent and the other a little bit bent in the back. It's up to you and it's all about personal preference.
What's very important about this trick is that you have enough speed so you can actually continue your manual as you go along. It's very difficult to do a manual while you're in place unless you're a bit more of an advanced skater, so I suggest having enough speed, but not too much that you might hurt yourself as you're doing this trick. Now, some people have a problem falling backwards or falling off to the side or the board getting away from them if they're going too fast. I suggest you take it at a medium pace and you feel comfortable enough to do this trick before you go it at a higher speed. And that's how you do a manual.