If you decide to write code for fldigi here is a very simple guideline for code structure

    https://sourceforge.net/p/fldigi/wiki/dev-docs/

Development, including compiling and linking is best when performed in a Unix like environment.
That includes Linux, Free-BSD, and MacOS.  It is possible to develop on a Windows OS, but I do 
not recomment doing so.

Most of the fldigi developers use Geany for coding and the command line interface for building
and debugging binaries.

All modems are derived from the virtual base class modem.

dave@tk7:~/SF/fldigi.git$ ls src
ale            cw            flarq-src      kml          mt63        rtaudio    waterfall
blank          cw_rtty       fldigi         libtiniconv  navtex      rtty       wefax
cmedia         debug         fldigi-main.o  logbook      network     soundcard  widgets
combo          dialogs       fldigirc.rc    logger       olivia      spot       wwv
common.rc      dominoex      flsynop.rc     main.cxx     openmp3     ssb        xmlrpcpp
compat         dtmf          fmt            Makefile     packet      stamp-h1
config.h       dxcluster     fsq            Makefile.am  psk         status
config.h.in    feld          globals        Makefile.in  psm         synop-src
config.h.in~   fft-monitor   ifkp           mbedtls      qrunner     thor
config_script  fileselector  include        mfsk         rigcontrol  throb
contestia      filters       irrxml         misc         rsid        trx

Blank contains blank.cxx and blank.h, outline class definition for any future added modem types.
Use them as a starting point.  Suggest studying psk.cxx and psk.h and comparing to the blank 
class code.

The new modem type will need to be added to both modem.cxx and modem.h   src/dialogs/fldigi.cxx
will need to be modified to add the new mode to the menu system and for linkage to modem 
initialization.

Configuration UI is in the very large source files, confdialog.cxx and confialog.h.  These are 
not maintained by hand coding!  The UI library, fltk, has a simple gui builder called fluid.  
The UI structures for fluid are in files with the extension .fl   From the top level source 
directory:

    fluid src/dialogs/confdialog.fl
        make UI changes
        exit fluid
    make flgen to create
        src/dialogs/confdialog.cxx
        src/include/confdialog.h

Modem specific configuration items are easily added to the file src/include/configuration.h  
Very few modem types require any hand coding in src/misc/configuration.cxx

The new modem may require a different style of user interface, for example FSQ, IFKP, 
FMT (frequency measurement test).  This is coded in the file src/dialogs/fldigi_main.cxx.  
Note that there are two principal fldigi user interfaces, (1) full UI with all controls 
available, (2) waterfall only UI with a very limited set of controls.  The additional 
modem specific UI must address both.  Each UI has its own set of menus.  All UI source 
is hand coded, so you must become intimate with the fltk widget types.  fltk is a C++ 
library.

All functions which modify the UI behavior, writing to a widget, changing it's UI size, 
location, color, accessibility, etc must be made from the UI thread which is managed by 
the fltk library.  You cannot make those calls directly.  Doing so will eventually cause 
a segmentation fault.  Modem code runs in a separate audio streaming thread (fldigi can 
have more than 20 concurrent threads).  Communication between the modem code and it's 
widgets is accomplished using a qrunner helper using macros.  See qrunner.h and check 
out various modem class code for instances of REQ, for exampe:

dave@tk7:~/SF/fldigi.git/src$ rgrep REQ fmt
fmt/fmt.cxx: REQ (clear_unk_scope);
fmt/fmt.cxx: REQ (clear_ref_scope);
fmt/fmt.cxx: REQ (put_unk_value, msg1);
fmt/fmt.cxx: REQ (put_unk_amp, msg1a);
fmt/fmt.cxx: REQ (put_unk_value, "");
fmt/fmt.cxx: REQ (put_unk_amp, "");
fmt/fmt.cxx: REQ (put_ref_value, msg2);
fmt/fmt.cxx: REQ (put_ref_amp, msg2a);
fmt/fmt.cxx: REQ (put_ref_value, "");
fmt/fmt.cxx: REQ (put_ref_amp, "");
fmt/fmt.cxx: REQ (set_fmt_scope);

A formidable task for the first time modem developer.  I am always available for consultation.

Suggest you clone the git repository for the latest code base.  When you are ready to be a 
contributor I can provide you with the ability to push changes to the remote code base.

    git clone git://git.code.sf.net/p/fldigi/fldigi fldigi-src