Have you read this entry in FileMaker help?
See "Editing record access privileges" in FileMaker Help and check out this particular sub section: "Entering a formula for limiting access on a record-by-record basis" for a description of how to set this up.
Note that it is very easy to commit a record prematurely. An alternative approach might be to use a form consisting of nothing but global fields for entering data for a new record. A "save" button on this form would then create the new record and copy over the data from the global fields to the fields of the new record. This table could then just be set up as Read Only for your users instead of using a Lock Expression.
If you want a little more field level control on preventing accidental modification of fields, you can try the modular Field Protection triggers on ModularFileMaker.org: http://www.modularfilemaker.org/module/field-protection-triggers/
They are simple and are designed to prevent accidental modification of data but allow deliberate changes. They could easily be modified to prevent changes once data exists in a field.