How do I create a long text for a document?

Please refer to the important parameters that are discussed in the previous question of reading texts. We will need the same parameters for saving the text as well. The function modules that you can use for this will be SAVE_TEXT followed by COMMIT_TEXT. The parameters play a very important roles as you might not see the saved text in the standard transaction if you give wrong parameter values.

CALL FUNCTION 'SAVE_TEXT'
EXPORTING
*   CLIENT                = SY-MANDT
  HEADER                =
*   INSERT                = ' '
*   SAVEMODE_DIRECT       = ' '
*   OWNER_SPECIFIED       = ' '
*   LOCAL_CAT             = ' '
* IMPORTING
*   FUNCTION              =
*   NEWHEADER             =
TABLES
  LINES                 =
* EXCEPTIONS
*   ID                    = 1
*   LANGUAGE              = 2
*   NAME                  = 3
*   OBJECT                = 4
*   OTHERS                = 5
        .
IF SY-SUBRC <> 0.
* MESSAGE ID SY-MSGID TYPE SY-MSGTY NUMBER SY-MSGNO
*         WITH SY-MSGV1 SY-MSGV2 SY-MSGV3 SY-MSGV4.
ENDIF.

CALL FUNCTION 'COMMIT_TEXT'
* EXPORTING
*   OBJECT                = '*'
*   NAME                  = '*'
*   ID                    = '*'
*   LANGUAGE              = '*'
*   SAVEMODE_DIRECT       = ' '
*   KEEP                  = ' '
*   LOCAL_CAT             = ' '
* IMPORTING
*   COMMIT_COUNT          =
* TABLES
*   T_OBJECT              =
*   T_NAME                =
*   T_ID                  =
*   T_LANGUAGE            =