The methodology I have used to successfully create models for SF follows. Your mileage may vary.
I cannot teach you how to model or make graphics, but I can tell you the programs I used and the proceedures to get the files you need.
Anim8or is a free download at
http://www.anim8or.com
Get the free Version of LithUnwrap at
http://files.seriouszone.com/download.php?fileid=198
In Anim8or:
View >All
Create a box or other shape
Tip: Double-click the object to set size (and to add subdivisions for a box).
A Length of 1000 (Z-axis) is about the size of a destroyer, but this size is unwieldy in Anim8or; use 1/10 scale (length of 100). We'll scale it up before exporting later (you can also rescale in SESF SpaceObjects_XFiles.txt).
Create the model (the hard part).
Tip: Build >Convert to Mesh.. to edit shape using object editing tools.
Tip: Options >Grid: Enable Snap.. to Grid with Spacing of 5.
Save your model.
When it is ready:
Edit >Rotate >Rotate Custom >x-axis: enter -90 (or you can rotate in SpaceObjects_XFiles.txt).
Click the UV button in the toolbar to make the yellow ring appear around your model.
Edit >Scale >Scale Custom: 10x (this will fill the screen with the model, and you may want to re-adjust the camera views).
Object >Export.. model from anim8or as 3ds file.
In LithUnwrap:
File >Model >Open.. your .3ds
Select >All
Tools >UV Mapping >Box (check apply spacing)
If necessary:
View >Zoom >Free
Edit >Scale >Free.. to fit the model map into the square.
File >UV Map >Save.. your bitmap
Expand the Material branch in tree in right panel and double-click default material;
(Material Properties) Select Bitmap under Diffuse Map, click Properties;
(Diffuse Map Properties) click change, select your bitmap.
File >Model >Save.. your model as .x (text, uncheck compressed).
In your Text Editor:
Edit the SpaceObjects_XFiles.txt file to give the ship the proper hardpoints and engine glows.
(so far, this as been trial and error for me)
In your Paint Program:
Change bitmap to suit.
Reload model in LithUnwrap and Preview.
Once the model looks as you prefer, copy the .x file to the XFiles folder of your mod, and the texture .bmp file to your Images folder. You now have a new ship! You'll need to edit the ship data files, of course (such as the Main_PurchaseShipsPlayer.txt or Main_StartingPlayerShips.txt, at least). Good luck!
I copied a lot of other ship text files to make the ship playable. SJ's Slot Modding Utils is really useful here.