Note that this only moves the data from a file with the .fmp12 format into a file with the .fp7 format. It does not move your layouts, scripts, etc into the .fp7 file.
What you can do is export the data (except for data in container fields), into text files. Merge files make a good choice as you get the field names along with the data. You can then open the .fp7 file and import this data. Don't know if the older version supports the "new table" option or if you have to create the table and then import into it.
You would need to do this for every table in your file.
You can export container files into a folder and if they are text or image files, you can use the Import Records | Folder action to import the data, IF Filemaker 7 supports that option. I don't recall when some of these features were first introduced so you will need to review the capabilities of FileMaker 7 before you try any of this.
Note that FileMaker 7 has a very limited life expectancy as software goes. It will work on fewer and fewer computers as OS versions are updated in Mac and Windows.
If you do not have any repeating-fields you may consider using an xml-export in the FMPXMLRESULT-grammar for all of your data except for the containers. (as Phil already pointed out).
Filemaker 11 supports "new table" option and that way the field-formats are identical, because that information is contained in the XML, where a normal text (which the merge-format actually is) doesn't.
If you have images in containers, you should add an extra field in your fmp12 db that will contain a unique name (like the recordnumber & original filename) and export all images to new files with the name in the field you just made. After this export to the xml and import the xml in fm7-fm11. Create a container-field there and start importing the images in the corresponding records.