While there is Borf's '3ds to RSM' converter tool, Borf has it added to Borf edit so use the one there as it is likely to be updated. Making a model that can be converted to RSM format is another issue. Someone who has done this can fill us in.
What You Will Need
- 3d Studio Max (Or something that can save to 3ds files)
3ds to RSM
Now the 3ds to RSM is a bit limited due to the fact the object must be fully textured and the textures be with the file (If your missing any it will make the tool list them as .bmp) and once converted to rsm the images have to be flipped upside down on the vertical axis and saved as bmp files... Ask Borf about that if you want more info. While with these limitation the tool is easy to use, simply open Browedit, load a map (It won't do anything until you do), and under 'Tools > Convert 3ds to RSM' select your 3d Studio Max file and click 'open'. The plug-in will then create a .rsm file which you can then add to Browedit (add it to the model text file or your own model text file) and then to your maps.
RSM to 3ds
To date there is nothing to convert RSM files to 3ds files.