Lets assume that you are using a pre-printed stationary, and wanted data alone to be printed by oracle on the checks... in which case you have to create an XML template considering the space/alignment/formatting requirements of your physical stationery...
You could start by downloading an system provided (Seeded) RTF template, and modify as per your liking and test it with the payment...
For downloading a standard check format RTF, perform the below steps...
Switch to Payables Responsibility
Navigate to Setup > Payment > Payment Administrator
Click "Go TO Task" icon next to XML Publisher Format Templates
Search for the name "Standard Check Format" and click GO ..
Click on the search result ..
Scroll down to see a download icon, by clicking on it , you can dowload and perform the modifications ..
You can download the step by step instructions with screen shots from the below link ......