Depends on the application, but in general you have to do these steps:

1 - Adapt the UI for Maemo (finger friendly UI, less buttons,..)
2 - Compile and package the application for Maemo.

Seems easy but for complex applications is a lot of work, you can find more information in these links:

http://wiki.maemo.org/Documentation/...eveloper_Guide
http://wiki.maemo.org/Documentation/...rting_Software