Some people use modal pages to create modal windows, but in some cases it is recommended to use the modal region for it. This solution will take advantage of some of the features found in the APEX 5 Universal theme because it is easier to Develop and debug.
1. Create a new region and than set this template to inline dialog. On my test i named it to Dialog Region.
2. On my example the modal will be open by a button click, so we need to create a button on another region.
3. Create a dynamic action which fires on button click and point to the created button.
5. Now you have to create a button to close the modal. Create it on the modal region and do the same as above but now with the following code.
6. Refresh the page and test it.