1. In menu File->Open File or Project (Ctrl+O);
2. In open file dialog find CMakeLists.txt in the root of QGIs
(/usr/local/src/qgis_trunk/CMakeLists.txt) and open it;
3. Run cmake in CMake Wizard. You can run it after adding project for
this you should navigate to cmake build directory it is qtcreator-build in
project root and run ccmake .. like usually);
4. After that QtCreator adds the new project with name qgis-1.x.x.
5. If you need to correct cmake options you can do it now (see 3);
6. Compile QGis (Ctrl+B);
7. Switch to tab Projects. In section Build Settings->Build Steps push
Details button and add in Additional arguments: install.
8. In section Run Settings push +Add and select in combo box Custom
Executable then push Details and add in Executable:
/patch/to/qgis/executable (/usr/local/bin/qgis);
9. Select Active run configuration on "Run /usr/local/bin/qgis"
10. Start Debugging QGis: F5.