How the ads look like, where they are displayed and what text you should write is actually very well explained by Facebook here. There are a couple of things though that are worth adding.
Maximum 20% text rule
This rule confuses people. It says that maximum 20% of the image can be text. The more confusing part is about measuring. But there is a solution for this. Facebook has a grid that goes over the image. You can do it yourself: upload the image, check the grid, click on the tiles that contain text and there you go: your measurement of how much of your image is text. Upload and test your image here. Update: Facebook has recently lifted the max 20% text ban, but to be on the safe side is good to anyway respect this rule. It’s not a bad rule in any case.
A piece of advice: don’t discard any ad placement from the beginning. Don’t turn of mobile ads just because your website is not mobile optimized or the right column ads because they look too small to be effective. This way you might unknowingly limit your reach. Turn them on and monitor the performance. After you get some learnings you can make informative decisions.
Facebook ads and online ordering
Many times you will have ads promoting a certain dish, a special discount, a lunch offer or just the menu itself. Here you should have a link going directly to your online menu, aiming to get online orders along with the potentially increased appetite of your viewers. Make sure you use the smart link that we provide in the Admin Area for these posts. The benefit is that no matter from what device the link is opened, browser or app, the menu will be displayed beautifully in desktop or mobile version, depending on the case.
This link also works fine with Facebook Ads, so go ahead and use it to optimize for clicks.