A fixed length record will have a key field. This is the main field of the record and is used to perform any ordering operations. A sequential file is a file which stores all records in key order (either ascending or descending but assume ascending unless told otherwise).
Reading from a sequential file does not change and you must still perform a serial search. Read the search page to find out how this is done.
Inserting a record into a sequential file is much different. As the record must be placed at a certian point you will have to perform the insertion alorithim. Read the page on insertion to a file for info on how this works.